In laymen terms, it takes just as much energy for a bit to be a 1 or 0 in memory, so there is no power savings by killing background processes. What Android does is suspend them so they remain dormant in memory until called upon again. When that happens, less processing power is needed to recall that data than to initialize it from scratch.
There are so-called memory cleaner/booster apps that will kill background processes, but they work counter to how Android is designed. When they kill a process, the OS will replace it to keep memory full. The cleaner app kills those and the cycle repeats. All that killing and restarting apps and processes result in increased power usage, not power savings.
Android generally does well enough on its own to not need meddling with memory.
As for your OS taking up so much, that does seem high. So my guess is you either have some corrupted data, the OS is poorly optimized, or it's reporting total usage and not just the core OS. I've never used OnePlus, but my LG V60 is currently only using 4.8 GB total, and just under 2 GB for the OS.