SDL's support strategy for Tridion is very similar to what most software companies do. They support their products on the versions of the platforms they have tested them on. It would be ridiculously expensive to test their products on the entire matrix of possible platforms, so they usually specify the current version and the one before the current one at the time of testing and release. In practice, this means that a given version of Tridion will be supported on only two versions of Oracle, two versions of MSSQL, two versions of Windows and so on.
Usually the supported versions remain the same for a service pack, but it is also possible that with a service pack, additional support is introduced. You should check with SDL what the road-map is for 2011 (To be honest - I suspect there won't be a SP2.)
I don't know of any failure scenarios for using this version of Tridion on an unsupported platform. It's quite likely that you are alone in considering doing this. The majority of customers always run on supported platforms, so the likelihood of other people finding the problems in advance for you is probably smaller than you should be comfortable with. Some customers are forced by circumstances to run on unsupported platforms, and in practice, customer support are often helpful, but if you encounter a problem that needs a hotfix, your luck will run out very quickly if you aren't on a supported platform.
I understand why you are seeking information to support delaying your Tridion upgrade, but you should also be seriously considering upgrading. The question you should (also) be asking here is "What problems do I face when upgrading to 2013?". As far as I know, 2011 to 2013 upgrades usually go well. Upgrading Tridion as well as Oracle may be the most straightforward route forward.