Hi,
I tested in my environment(Outlook 2019) and can successfully change the attachments save path. Please make sure you have followed the method in the article step by step.
- Close your Outlook client.
- Press Windows key + R to open the run command then type regedit and press OK to open the Registry Editor.
- Go to HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Options.
- Right click on Options and create a new string value called
DefaultPath.
- Type the path you want in the value of the DefaultPath.
- Waite a minutes, start your Outlook client and right click the attachments > Save as, then the default path should be changed.
(Important: Serious problems might occur if you modify the registry incorrectly. Before you modify it, back up the registry for restoration in case problems occur. )
Besides, if you'd like to directly save your attachments in your Onedrive. Just right click the attachments > Upload/Upload All Attachments.

Hope this can be helpful.
Regards,
Aidan Wang
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