OpenACS is a 95% port of ACS/Oracle and, excluding the Java-Oracle specific modules (e.g. webmail) we should reach 100% really soon now (R).
OpenACS is not a subset of ACS/Oracle. We intend to provide full capabilities, and we hope more and more modules are created by OpenACS users.
Even the Java modules are being worked on by Dan Wickstrom and we can expect them to be ported as well.