TwoFace,
what is your prior experience/tolerance with benzo's, opiates, and cocaine? All of these are important to know.
I personally don't think that combining cocaine with any depressant is that great of idea, unless its your last line/shot. If you've got 5 or so lines cut out, taking the oxy or xanax along with the cocaine is really going to be a waste, or possibly lead to taking dangerously high dosages of benzo's/opiates, as you're going to want to use a depressant to aid in the crash as well (which is why I suggest either saving the oxy or xanax for the last line/shot, or better yet, wait until you start to crash and then dose your opiate or benzo.)
I also think from a HR perspective, alprazolam, or benzodiazepines would be the better choice as they lower your chance of a seizure, while cocaine increases your chances.