The solubility for oxycodone HCl is roughly 100gmg per mL, with a slight variance.
When intravenously administering oxycodone pills, you have to remember that a HUGE percentage of the pill is actually binders and fillers and other insoluble junk, which you do not want in your syringe/veins.
So going by this, fundamentally you should be able to get three Roxi 30s into 1mL of water. Now due to the other inactivees and binders and whatnot that get left behind, those can hinder the optimal absorption you are aiming for.
But down to brass tacks, if you are trying to inject ANY kind of pill, it is IMPERATIVE that you invest in some micron filters. They are worth the couple of bucks they cost, and your veins/body will thank you. Using those, you can get rid of all binders/fillers and come back with a practically clear solution. Using microns, you can also get closer to the optimal solubility and shouldn't hvae a problem fitting 90mg of oxycodone in a 1mL syrince.