Just a quick note. Test out PHP/Oracle combination, and make sure its stable before stopping mysql.
The last time I dealt with PHP/Oracle, it had some rather nasty side affect that made me switch to mysql.
Two things to look for:
(1) number of openfiles. Keep the site running with Oracle, and do something like
lsof | grep oracle | wc -l
to see how many open file there are at the moment.
(2) number of zombie or sleeping process. Do something like
ps -aux | grep oracle | wc -l
and keep an eye on how many process there are for the oracle user.
At least for me, when I stop one problem, the other one comes up.
-Will