Issuing status update on behalf of user


I have an application which has successfully retrieved an access token for a particular user using: oauth/access_token
I would then like to have the application post a status update on behalf of the user (presumably using the retrieved access tokens), however, whenever I try to do this I get a Bad Request 400.
It seems from the documentation that this may not be possible because there is no “user context”.

Is it currently possible to perform status updates on behalf of a user?

Do I need a bearer token, maybe, from: oauth2/token to do this?