tolerance is a must!! aand you have to know what you are doing. if you dont have experience with these drugs, you could fuck up and die. however from what you are saying, you have a tolerance to benzos and opiates, and you probably know your limits . a good piece of advice for combining drugs is to cut your normal dose in half for all drugs.
for example, if you usually take 2mg ativan alone, and 40-80mg alone, then the safest action to do would be to only take 1mg Ativan, and 20-30mg oxycontin. i would keep the concerta dose low it is a shitty drug, but the dopamine release from each drug will synergize and give you a fuckkkkkk hahahah, you only need a tiny bit. i would say 5-10mg ritalin with your normal dose of oxy, you will feel great energetic euphoria where you body feels like you just had sex and stoned. if you want, you can take some ativan with that too, but only take a low dose or you will ruin the oxy and ritalin. id say like only .5mg ativan with 15-20concerta and 30mg oxy. then after 2-4hrs take another 0.5mg ativan.. then when you want to come down, take 1-2mg ativan . holy shit that combo makes my mouth water, and you since those doses are low and you are tolerant, you will fine. just dont drive!!! stay at your house, clean, get on bluelight, ask a friend to pick you up and hang out, but dont drive!!! after