I had trouble remembering why FBI Agent O'Connor and Mary despised each other so much (since Mary gives almost all outside agents/officers a hard time LOL). So I looked up which episodes Agent O'Connor guest-starred in, then found recaps to bring me up to speed on their history, so I'll link them here in case anyone else wants to brush up.
Stan By Me 1x11
A Fine Meth 1x12
In My Humboldt Opinion 2x02
Who´s bugging Mary? 2x09
The Medal of Mary 5x06
Sacrificial Lam 5x07
All that's left now is the finale, hopefully that mean Agent O'Connor will get his just deserts!
(BTW: Looking all this up, I learned that the actor playing Agent O'Connor is Mary's RL brother LOL)