I'm playing in an OA game that started under 3.0 but made the switch to 3.5 The transition came off without a hitch and so far things have been running smoothly.
The update to the Mnk class was a godsend, most of the other PHB classes aren't used.
The feat changes were integrated without a hitch. (except for the fact that some already shared the names of new 3.5 feats, little bit of confusion there)
OA does not make use of many common DnD spells which were changed (ie: disentegrate) so that was a non-issue. Most of the new spells in OA are designed to do what they do for flavor reasons, and thus don't duplicate effects of save or die spells etc, so we left them alone. I do not anticipate may spell rewrites in OA 3.5. Same thing with magic items.
Most of OA's variant rules (ie: ijatsu focus, martial arts styles) dont relate to core mechanics and so I see no reason why they would change.
In short, OA is really more of a "flavor template" that is built on d20 rather than a variant system. As such if you already understand 3.5 DnD rules then you should have no problem running a 3.5 OA.