I cannot remember what put O'Brien and Thomas against each other. Way back O'Brien mistakenly thought Cora was going to replace her. Cora was pregnant (possibility of male heir) and O'Brien deliberately placed soap on the floor to cause Cora to slip as she stepped out of tub. (She is Cora's lady's maid) Cora had a miscarriage and O'Brien discovered Cora had been looking for a lady's maid for someone else.

O'Brien also wants to clear the way for her nephew Alfred to move ahead in the household staff. If Thomas leaves, there would be a higher level vacancy which could allow him to move up, and shake up the number of male servants so that Alfred would have more seniority.

I think maybe O'Brien holds Thomas responsible for the soap incident. They were always baiting each other and egging each other on in their evilness. The phrase Bates whispered was "her ladyship's soap".

I was so hoping we would finally get rid of the dreadful Thomas. O'Brien is still pretty rotten, too.

When Alfred who is O'Brien's nephew first was hired O'Brien asked Thomas to help him out and Thomas refused and that really set O'Brien against him. Each are really only interested in themselves.
Joann

The villians make the heroes more heroic and make the storyline more interesting.

Thanks, all. And I actually felt sorry for Thomas last night. I think Obrien is much more evil, though, but they are both bad.