The Oracle version should be the same on both servers.
This isn't about Oracle versions so far as I can tell. I think the confusion may arise from the acronym OS. I assume that the OP means "Operating System"; I suspect you are interpreting it as "Oracle Server".

So far as I know it is not possible to restore an Oracle database from one operating system to a different one. On the other hand 10g cross-platform transportable tablespaces do allow this kind of migration where supported (although this has nothing to do with RMAN).

As a point of interest, Oracle do seem to support RMAN across Oracle versions where major version is the same and source minor version is < target minor version. I recently needed to clone a 9.2.0.6.0 database to 9.2.0.7.0 server install and contacted Oracle support. I was told this was fine (providing I ran catpatch after the clone of course). I suspect that tthe same would apply to disaster recovery as well. Anyone care to comment?