I have some PPA's in my update sources that don't maintain active builds for oneiric. Is it safe to continue receiving updates from these PPA's using builds for prior releases like Natty or Maverick? Thanks.
In general installing a package from a different distribution isn't recommended as explained in the pinning how to wiki page.
However, you can still try to:
If your sources are correctly configured this this can be done with:
All this is explained in the recommended alternative to pinning section in the how to. Please note, that the assumption of the how to is that you try to install packages from a new distribution in an old one, not the other way around; but I think most of the contents there is still valid to your case.
Finally, for some particular cases, like applications that don't try to modify your system, written in an interpreted language, then probably you're safe to install the software directly from the PPA.