I wouldn't say Pikachu OHKOs a bunch, but it definitely gets a lot of KOs in general.
Why? Well, there are a lot of theories for why Pikachu is OP. I'll just say what I think.
Starting with Pikachu getting KOs, Ash (from my experiences with the anime) will send out a Pokemon, weaken the opponent, then his Pokemon will faint. He'll then bring out Pikachu to finish the opposing Pokemon off, and then he'll repeat the process later in the match (assuming Pikachu is tired, which sometimes takes a few KOs for that to happen).
As for why Pikachu is just very OP in general, I'm going to say that it Pikachu's strong bond with Ash, and the fact that it has gone through so many battles and trained quite a bit. Pikachu has been with Ash since his journey first started, and while one of the most inconsistent Pokemon that Ash has had in his possession (though the Snivy battle is brought up way too much. I believe in the Pikachu Got Struck By Lightning So Its Level Reset theory, plus Pikachu lost its ability to use Electric moves due to Zekrom.), Pikachu has an undeniably strong bond with Ash, and if you think about it, the anime is all about trainers and the bonds they share with their Pokemon. Throughout every season of the anime, Ash and Pikachu have been together, training and becoming stronger together.
Also anime logic, lol.
Hope this helps! :)