OAuth is web standard to manage permission and security for modules like App on web. in simple word OAuth protocol is used to authenticate and authorize apps and service. in order to use context token you need OAuth.
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To authorize requests by an app for SharePoint to access SharePoint resources on behalf of a user.
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To authenticate apps in the Office Store, an app catalog, or a developer tenant.
For example, OAuth is used in cases where calls have to be made from a remote web server to SharePoint 2013 on behalf of
a user. You would not use OAuth to make a call from the app web or from a remote webpage using the client library, for example.
To access resources, an app has to request app permissions. In general, delegated authorization codes or access tokens
are issued by the OAuth security token service (STS). An example of OAuth STS is Windows Azure Access Control Service (ACS) OAuth endpoints. In contrast, the WS-Federation STS and the Security Assertion Markup Language (SAML) passive sign-in STS are primarily
intended to issue sign-in tokens.
see OAuth.net and Web
Authorization Protocol (oauth).
http://msdn.microsoft.com/en-us/library/fp142384.aspx
hope so it helps
