the difference is oxycontin is an extended release tab (over 8-12 hours) oxycodone hcl is an immediate release form of the drug-it's effects last 3-5 hours, depending on how your body processes it
If your pain is constant, you should be on oxycontin every 8 or preferably, every 12 hours. The ER dose won't necessarily be the total mg dose of the IR dosage you take now. You may have to adjust the dose every 3-4 days until your Dr finds what is right for your metabolism and tolerance