What's the difference between Windows Automated Installation Kit (AIK) and OEM Preinstallation Kit (OPK)?

1. Target audience, Terms and Licenses (obviously). See more on this on relative product official pages.

All the comparison below is based only on file contents between two medias and does not include differences in folder names, shortcuts and/or registry entries.

2. OPK DVD is bootable and thus it includes the following files and folders. AIK DVD does not—and that's why its media is quite smaller.

.\bootmgr
.\boot\bcd
.\boot\boot.sdi
.\boot\bootfix.bin
.\boot\bootsect.exe
.\boot\etfsboot.com
.\boot\fonts\chs_boot.ttf
.\boot\fonts\cht_boot.ttf
.\boot\fonts\jpn_boot.ttf
.\boot\fonts\kor_boot.ttf
.\boot\fonts\wgl4_boot.ttf
.\efi\microsoft\boot\bcd
.\efi\microsoft\boot\fonts\chs_boot.ttf
.\efi\microsoft\boot\fonts\cht_boot.ttf
.\efi\microsoft\boot\fonts\jpn_boot.ttf
.\efi\microsoft\boot\fonts\kor_boot.ttf
.\efi\microsoft\boot\fonts\wgl4_boot.ttf
.\sources\boot.wim

3. Branding. The following files are different.

.\background.bmp
.\startcd.ini
.\waik_license.rtf

(The last file is obviously located only on the AIK media and not in the OPK).

4. Documentation (also obviously). Here's the difference in what's provided with both kits. A lot of AIK documentation is available online and is not included into the media.

AIK OPK
.\readme.htm .\readme.htm
.\docs\chms\waik.chm .\docs\chms\opk.chm
.\docs\chms\usmt.chm
.\docs\whitepapers\stepbystep_itpro.htm .\docs\whitepapers\stepbystep_oem.htm
.\docs\whitepapers\system_builder_quick_start.htm
.\docs\whitepapers\hardware requirements\media center\green_button_art_rbg.png
.\docs\whitepapers\hardware requirements\media center\ime_localization.xls
.\docs\whitepapers\hardware requirements\media center\mcb_11mm_universal_keycap_asm.eps
.\docs\whitepapers\hardware requirements\media center\mcb_11mm_universal_keycap_asm.stp
.\docs\whitepapers\hardware requirements\media center\mcb_6_6mm_universal_keycap_asm.eps
.\docs\whitepapers\hardware requirements\media center\mcb_6_6mm_universal_keycap_asm.stp
.\docs\whitepapers\hardware requirements\media center\mcb_9mm_universal_keycap_asm.eps
.\docs\whitepapers\hardware requirements\media center\mcb_9mm_universal_keycap_asm.stp
.\docs\whitepapers\hardware requirements\media center\mediacenterinternationalsupport.pdf
.\docs\whitepapers\hardware requirements\media center\mediacenterinternationalsupport.xps
.\docs\whitepapers\hardware requirements\media center\ocur_deployment_guidelines.pdf
.\docs\whitepapers\hardware requirements\media center\ocur_deployment_guidelines.xps
.\docs\whitepapers\hardware requirements\media center\printed_thin_profile_button_art_cmyk.eps
.\docs\whitepapers\hardware requirements\media center\rc-ir-collection.pdf
.\docs\whitepapers\hardware requirements\media center\rc-ir-collection.xps
.\docs\whitepapers\hardware requirements\media center\wmc_opk_intro.pdf
.\docs\whitepapers\hardware requirements\media center\wmc_opk_intro.xps
.\docs\whitepapers\hardware requirements\media center\xp_ir_trans_rec_req.rtf
.\docs\whitepapers\hardware requirements\tablet\vista_mobile_pc_technical_guidance_whitepaper.pdf
.\docs\whitepapers\hardware requirements\tablet\vista_mobile_pc_technical_guidance_whitepaper.xps
.\docs\whitepapers\software customization\oem_user_experience_guidelines.pdf
.\docs\whitepapers\software customization\oem_user_experience_guidelines.xps
.\localized_docs\bg-bg\oem_user_experience_guidelines.pdf
.\localized_docs\bg-bg\oem_user_experience_guidelines.xps
.\localized_docs\bg-bg\system_builder_quick_start.htm
.\localized_docs\et-ee\oem_user_experience_guidelines.pdf
.\localized_docs\et-ee\oem_user_experience_guidelines.xps
.\localized_docs\et-ee\system_builder_quick_start.htm
.\localized_docs\hr-hr\oem_user_experience_guidelines.pdf
.\localized_docs\hr-hr\oem_user_experience_guidelines.xps
.\localized_docs\hr-hr\system_builder_quick_start.htm
.\localized_docs\lt-lt\oem_user_experience_guidelines.pdf
.\localized_docs\lt-lt\oem_user_experience_guidelines.xps
.\localized_docs\lt-lt\system_builder_quick_start.htm
.\localized_docs\lv-lv\oem_user_experience_guidelines.pdf
.\localized_docs\lv-lv\oem_user_experience_guidelines.xps
.\localized_docs\lv-lv\system_builder_quick_start.htm
.\localized_docs\ro-ro\oem_user_experience_guidelines.pdf
.\localized_docs\ro-ro\oem_user_experience_guidelines.xps
.\localized_docs\ro-ro\system_builder_quick_start.htm
.\localized_docs\sk-sk\oem_user_experience_guidelines.pdf
.\localized_docs\sk-sk\oem_user_experience_guidelines.xps
.\localized_docs\sk-sk\system_builder_quick_start.htm
.\localized_docs\sl-si\oem_user_experience_guidelines.pdf
.\localized_docs\sl-si\oem_user_experience_guidelines.xps
.\localized_docs\sl-si\system_builder_quick_start.htm
.\localized_docs\sr-latn-cs\oem_user_experience_guidelines.pdf
.\localized_docs\sr-latn-cs\oem_user_experience_guidelines.xps
.\localized_docs\sr-latn-cs\system_builder_quick_start.htm
.\localized_docs\th-th\oem_user_experience_guidelines.pdf
.\localized_docs\th-th\oem_user_experience_guidelines.xps
.\localized_docs\th-th\system_builder_quick_start.htm
.\localized_docs\uk-ua\oem_user_experience_guidelines.pdf
.\localized_docs\uk-ua\oem_user_experience_guidelines.xps
.\localized_docs\uk-ua\system_builder_quick_start.htm

5. OPK includes the following items that are not available in AIK.

5a. Microsoft Windows Recovery OPK Configuration Tool (WinReCfg.exe).

5b. The Windows Recovery Environment feature package for Windows PE (WinPE-SRT). It provides a recovery platform for automatic system diagnosis and repair and the creation of custom recovery solutions. It includes the following Optional Components (OC).

Component File name File Description
Microsoft-Windows-BootEnvironment-DVD-BOOTFIX - -
Microsoft-Windows-BootEnvironment-DVD-EFI BCD -
boot.sdi -
Microsoft-Windows-BootEnvironment-DVD-EFISYS efisys.bin -
Microsoft-Windows-BootEnvironment-DVD-ETFSBOOT etfsboot.com -
Microsoft-Windows-BootEnvironment-DVD-PCAT BCD -
boot.sdi -
Microsoft-Windows-BootEnvironment-DVD - -
Microsoft-Windows-DisasterRecoveryUI bmrui.exe Microsoft® Windows Disaster Recovery UI
Microsoft-Windows-ErrorReportingCore wer.dll Windows Error Reporting DLL
WERDiagController.dll WER Diagnostic Controller
wermgr.exe Windows Problem Reporting
Microsoft-Windows-ErrorReportingUI werui.dll Windows Error Reporting UI DLL
Microsoft-Windows-ICacls icacls.exe -
Microsoft-Windows-Memory-Diagnostic-Schedule MdSched.exe Windows Memory Diagnostics Tool
Microsoft-Windows-RecDisc-Main recdisc.exe Microsoft® Windows Repair Disc
Microsoft-Windows-SPP-Main spp.dll Microsoft® Windows Shared Protection Point Library
sxproxy.dll Microsoft® Windows System Protection Proxy Library
Microsoft-Windows-SRT - -
Microsoft-Windows-SystemRestore-Main srclient.dll Microsoft® Windows System Restore Client Library
srcore.dll Microsoft® Windows System Restore Core Library
rstrui.exe Microsoft® Windows System Restore
Microsoft-Windows-SystemRestore-SrHelper srhelper.dll Microsoft® Windows driver and windows update enumeration library
Microsoft-Windows-VssService VSSVC.exe Microsoft® Volume Shadow Copy Service
Microsoft-Windows-WinPE-OC-SRT-Deployment - -
Microsoft-Windows-WinRE-RecoveryAgent ReAgent.xml -
ReAgent.dll Microsoft Windows Recovery Agent DLL
Microsoft-Windows-WinRE-Tools StartRep.exe Startup Repair
RecEnv.exe Windows Recovery Environment
BootRec.exe Disk Metadata Diagnosis and Repair Tool (aka FixPart.exe)

6. Code samples. They are a bit different between the Kits. I have no idea on what all these files are about so here's just the list.

6a. Help Customization.

AIK OPK
.\f_sample_aik_helpcust_escalation.h1c .\f_sample_opk_helpcust_escalation.h1c
.\f_sample_aik_helpcust_escalation.h1v .\f_sample_opk_helpcust_escalation.h1v
.\f_sample_aik_helpcust_escalation.xml .\f_sample_opk_helpcust_escalation.xml
.\f_sample_aik_helpcust_escalation_assetid.h1k .\f_sample_opk_helpcust_escalation_assetid.h1k
.\f_sample_aik_helpcust_escalation_bestbet.h1k .\f_sample_opk_helpcust_escalation_bestbet.h1k
.\f_sample_aik_helpcust_escalation_linkterm.h1k .\f_sample_opk_helpcust_escalation_linkterm.h1k
.\f_sample_aik_helpcust_escalation_subjectterm.h1k .\f_sample_opk_helpcust_escalation_subjectterm.h1k
.\f_sample_aik_helpcust_f.png .\f_sample_opk_helpcust_f.png
.\f_sample_aik_helpcust_fabrikam.windows.client.f.png .\f_sample_opk_helpcust_fabrikam.windows.client.f.png
.\f_sample_aik_helpcust_fabrikam.windows.client.stylesheets.escalation.css .\f_sample_opk_helpcust_fabrikam.windows.client.stylesheets.escalation.css
.\f_sample_aik_helpcust_fabrikam.windows.client.stylesheets.helphome.css .\f_sample_opk_helpcust_fabrikam.windows.client.stylesheets.helphome.css
.\f_sample_aik_helpcust_helphome.h1c .\f_sample_opk_helpcust_helphome.h1c
.\f_sample_aik_helpcust_helphome.h1v .\f_sample_opk_helpcust_helphome.h1v
.\f_sample_aik_helpcust_helphome.xml .\f_sample_opk_helpcust_helphome.xml
.\f_sample_aik_helpcust_helphome_assetid.h1k .\f_sample_opk_helpcust_helphome_assetid.h1k
.\f_sample_aik_helpcust_helphome_bestbet.h1k .\f_sample_opk_helpcust_helphome_bestbet.h1k
.\f_sample_aik_helpcust_helphome_linkterm.h1k .\f_sample_opk_helpcust_helphome_linkterm.h1k
.\f_sample_aik_helpcust_helphome_subjectterm.h1k .\f_sample_opk_helpcust_helphome_subjectterm.h1k
.\f_sample_helpcust_corporate.h1c .\f_sample_helpcust_oem.h1c
.\f_sample_helpcust_corporate.h1t .\f_sample_helpcust_oem.h1t
.\f_sample_helpcust_corporate_assetid.h1k .\f_sample_helpcust_oem_assetid.h1k
.\f_sample_helpcust_corporate_bestbet.h1k .\f_sample_helpcust_oem_bestbet.h1k
.\f_sample_helpcust_corporate_linkterm.h1k .\f_sample_helpcust_oem_linkterm.h1k
.\f_sample_helpcust_corporate_subjectterm.h1k .\f_sample_helpcust_oem_subjectterm.h1k

6b. Mobility Center Customization. This is available in the OPK only.

.\f_mobility_icon1.ico
.\f_mobility_icon2.ico
.\f_mobility_icon3.ico
.\f_mobility_icon4.ico
.\f_mobility_icon5.ico
.\f_mobility_icon6.ico
.\f_mobility_icon7.ico
.\f_mobility_icon8.ico
.\f_mobility_mobilitycenterextensibility.idl
.\f_mobility_mobilitycenterextensibility_sample.idl
.\f_mobility_mobilitycenterrefresh.h
.\f_mobility_mobilitycenterrefresh.h_sample
.\f_mobility_mobilitycenterrefresh.lib
.\f_mobility_mobilitycenterrefresh_amd64.lib
.\f_mobility_mobilitycenterrefresh_ia64.lib
.\f_mobility_mobilitycenterrefresh_x86.lib
.\f_mobility_mobilitycentersample.cpp
.\f_mobility_mobilitycentersample.h
.\f_mobility_mobilitycentersample.rc
.\f_mobility_mobilitycentersample.reg
.\f_mobility_mobilitycentersample.rgs
.\f_mobility_mobilitycentersample.sln
.\f_mobility_mobilitycentersample.vcproj
.\f_mobility_oemlogo.png
.\f_mobility_resource.h
.\f_mobility_stdafx.cpp
.\f_mobility_stdafx.h
.\f_mobility_tilebutton1.cpp
.\f_mobility_tilebutton1.h
.\f_mobility_tilebutton2.cpp
.\f_mobility_tilebutton2.h
.\f_mobility_tilebutton3.cpp
.\f_mobility_tilebutton3.h
.\f_mobility_tilebutton4.cpp
.\f_mobility_tilebutton4.h
.\f_mobility_tiledropdown.cpp
.\f_mobility_tiledropdown.h
.\f_mobility_tileslider.cpp
.\f_mobility_tileslider.h
.\f_mobility_tilesliderlive.cpp
.\f_mobility_tilesliderlive.h

All other content is identical. Including .\Neutral.cab (that contains the vast majority of tools) and .\WinPE.cab (that is different for some reason but its contents is absolutely the same).