I am playing on Oled monitor and just wondering if put brightness to 80 or 100 just while gaming will that cause burn in issues or would it be safe
Prefacing that there isn't really a 100% sure answer since there just isn't enough definitive OLED burn-in tests yet.
Will vary between what make/model/brand you've got, particularly what anti burn-in features it ships with. Newer one with taskbar detection, better pixel refresh/shifting and such? Might be fine leaving it at 100%. At least for determining the "good enough to not RMA" life within the (to be fair, usually short) warranty period - follow whatever the manual says.
At least from what I've seen, mostly from RTING's durability tests, is that it's still a very large amount of hours to see permanent burn-in happen in the worst case scenarios for most OLEDs (100% brightness, static elements). Although you can get spooked by slightly longer lasting, but temporary image retention. Provided the protection features are working properly.
Regardless, if longevity is a pretty big priority, then 80% is ofc. going to just be better. Not too dissimilar from the phosphor coating of CRTs, it's a matter when not if that it'll simply be spent in some areas of the screen. The more hours per day you use it, the more relevant that is. If it's just shooting emails for some hours, and gaming for a couple hours, I'd leave it at 100% and enjoy life.