Actually, Sean isn't exactly right about forgetting Oracle. The currently completed version of OpenACS--3.2.5--does indeed use only PostgreSQL. There are other folks here who can tell you far better than I about how Postgres compares to Oracle, but the consensus seems to be that it is much, much easier.
That having been said, if you are absolutely in love with Oracle, OpenACS 4.x, which is currently under development, will run on both Oracle and PostgreSQL. But you'll probably have to wait a month or two before there's a critical mass of packages ported and ready to go on 4.x.
