How to completely un/reinstall Android OS, battery drain

A

AC Question

Hello,
I have OnePlus2 with android 6.0.1 oxygenOS. Since last month my batter drain is insane and it's due to android system (according to battery panel). No matter what I do with my phone, it's always number 1 power use.
I've tried downloading ROM, and installing it from built in memory, but it didn't help. (I also done all the data and cache wipe).
I guess it may be some sort of very well hidden malware.
May it be caused by a physical damage? The whole thing happened after I was running in rain and phone got a little wet sadly (for example the home button was pressing itself all the time and phone was vibrating cut of it, lasted for about 30-60mins, I just left the phone so I don't know exactly). Maybe the moisture damaged and now it drains power?
I attach some screen shots of my battery panel:
OK i can't do it..
In battery panel there are some weird apps called "99328" or "99345"? Also on this battery charge, Android system had 38% power use, and I was browsing net the whole time, chrome has 21%, then screen 11%, android os 7% and those weird apps 2% each.
I charged battery to almost full and then played only clash of clans for 2h. Lost 60% and still Android system had biggest battery drain, Android system 14%, clash 9%, screen 7% Android os 3%.
Please help, as I am close to RMAing the phone and losing it for 2 weeks :/