Dr. Dave Thomas is the Chief Surgeon at Princeton-Plainsboro Teaching Hospital. He appears in the episode 5 to 9. In addition to being the overall supervisor of all the surgeons at the hospital, he is responsible for the administration and allocation of all of the operating rooms.
In 5 to 9, we learn that at the beginning of Season 4, Lisa Cuddy convinced Dr. Thomas to hire Robert Chase for the hospital's surgical staff so that Chase could become board certified. However, we also learn that when Chase re-joined House's team in Season 6, Dr. Thomas was upset that Cuddy hasn't hired a replacement for Chase.
House likes to confront Thomas about the fact he stole Chase from his staff and is always looking for an opportunity to "rub it in" because House believes that Chase was the best surgeon Dr. Thomas had. At the end of 5 to 9 while House has maneuvered Chase into position to perform transplant surgery instead of using Dr. Hourani, who even House feels is well qualified, Chase and Dr. Thomas get into a physical altercation in the lobby that Cuddy has to break up.