Cpu sets fps. It pre-renders the game info and ships it off to the gpu. Gpu sets resolution and details and has to try and live upto what the cpu gives it. Consequently, there's always a bottleneck (if you want to call it that) because nothing is ever identical. If the cpu is capable of 300fps, and the gpu can only put up 200fps output, most would call that the gpu bottlenecking the cpu. Which it isn't because the best you can get is 165fps out of a 165Hz monitor. Change games to something more demanding on the cpu, and now it's only capable of 200fps, but if the gpu can do 300fps is the cpu now bottlenecking the gpu?
The whole idea of bottlenecking honestly should be flushed. You have a 9900k that's capable of high fps. You have a RTX2080 that's capable of reproducing high fps at good details. At 1080p the cpu will be taxed, the gpu relaxing, at 4k the cpu will be relaxing and the gpu taxed. At 1440p/144Hz they both be taxed, either/or dependent on the game.
Should you OC? Play the most cpu challenging game you own. Turn detail settings from ultra to medium. If the fps jumps up considerably, it's the gpu that's stopping the fps. If nothing changes much at all then it's the cpu not strong enough. And that'll be your answer. OC will help raise the fps to match what the gpu can put out. If the gpu is the issue, then OC on the cpu isn't going to do much of anything.
Either way 'should' is argumentative and a personal decision, but as powerful as your pc is, 'should' isn't a word that applies. OC if you want to, OC if you feel that you must, OC just because you can. All up to you.
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