Usually , we follow the as you metioned above as one as DB server and other as Application Server. Its the recommended Architecture setup.
some times OBIEE will installed in a seperate server as the OBIEE can use the complete utilization of the server.
Most of the ODI Load(Runs at a night time) and OBIEE(usage will be heavy in day time) will not occur at same time. So, the first one is the best/recommended approach.