Asana- though not designed for OKRs, would be a pretty simple place to keep them. Especially if you are already using it or something similar.
Word of warning- despite the current trendiness to talk about OKRs, I feel they do more damage than good if tied to bonuses. It's a rehashing of Drucker's Management by Objective (MBO) which comes out of the 50's and isn't generally used anymore.
Most roles- outside of perhaps sales- don't distill simply into quantitative metrics that are good enough to tie bonuses too. Instead, you get people sub-optimizing and doing damage to the business.
OK, I'm off my soapbox... :)