Mirtazapine will likely not do much compared to flexeril. Take a low-dose of flexeril first and allow for it to kick in if you want to potentiate the effects, but be really careful and do NOT overdo it. Also, do not mix the two substances if you don't already have a good tolerance to both. If you do not have a tolerance, just take the oxy. Insufflating 15mg if you're new to it will probably give you a noticeable chill buzz without knocking you out or making you feel super nauseous, but that also depends on genetic factors, weigh / height, obviously tolerance, and metabolism.
Whatever you do, I'd say start off slow. Break your oxy in half first, insufflate 7.5mg, then wait at least 20 minutes before you decide whether or not you want to do the second half. If 7.5 gets you good, or nauseous, you'll be able to save the other half for another time. This is the same type of harm reduction method I recommend to heroin users that IV when they get a new bag they're unfamiliar with - split your usual "shot" after cooking it up into 2 separate syringes to prevent OD from an unfamiliar batch. If it's strong, you just avoided death or a trip to the hospital. If it's normal / what you're used to, the other dose is literally right there.
