We currently use DBI to connect to Oracle 7.3.4.5 and 9.2 all the time, and so far, have not encountered any significant problems.
Our DBD::Oracle was compiled using the 7.3.4.5 oci but, we use the Oracle 9.0.1 tnslsnr. Oracle 9.0 seems to be a middle version that can communicate with a 7 and 9.2 database.
You've probably realized this if you have ever attempted to create a database link in Oracle 7 to try and access an object in Oracle 9.2. You can't. However, you can use a 9.0 database is a hopping point. We have many apps here that take advantage of newer database packages by doing this 7->9.0->9.2 database link trick. It's not exactly the most efficient process but if you're desparate like we were...