is this your first time taking opiates? are you scripted the 20mg oxycodone, and if so what form (OP's, perc's)? cause if they're OP's it'll release the 20mg's over 8-12 hours (releasing 10mg's instantly i believe, correct me if i'm wrong) and you may not be feeling all that high cause of the slow onset. another thing is that opiate euphoria is subtle, yet powerful, and is dependent on the type of opiate and which ROA and dose you decide to use. so you maybe expecting a different type of high you are getting. oxy has a high oral BA so swallowing them is better than any route. technically IV will give you 100% BA, but isn't really worth it IMO (I've tried it a few times).
as to your tolerance question. it depends on how you decide to use them. if you are using them for pain, and have a script i would just stay at your prescribed dose (if your dose isnt working for pain management then talk to your doc about a higher dose). that will keep your tolerance lower for longer. If you decide to abuse them for a high your tolerance will likely go up within a week. if you have OPs the only way to abuse them, without going through a shit ton of work that isn't worth it IMO, is to double up on your dose, but be careful doing that if you're not tolerant.. maybe take 1.5x your dose (30mg's).
tread carefully.. opiate addiction can be a long and dark road. some people can function, but most will lose a lot in their lives with addiction so be careful.