This is not well documented but we were able to do this via Forms Personlization and "Navigate to EBS" action framework configuration in OBIEE. You will need to work w a Oracle EBS developer to do the forms personalization to ensure that you pass the correct variable from OBIEE. Also, the authentication must FIRST be done in EBS->OBIEE and only then can you go back to EBS (via the session cookie). I do not believe it is currently possibly to go direct from OBIEE to EBS via the form..it may be theoretically possible via external table authentication but I have raised this question and noone has provided an answer as far as I know. Pls mark correct or helpful if this response helps.
We were able to make the authentication (as we use common OAM SSO server between EBS and OBIEE) and OAG page integration work. I just had to make sure that I pass the same parameters with which the OAF page is called. Also the OAF page needs to associated with a function which we use in OBIEE.