Strange.
Reports said that Bolton was fired because he was too hawkish on Iran, and/or because Trump disliked that Bolton was running his own little cabal inside the Executive.
It sounds like O’Brien is another hawk. So, why would Trump find that appealing?
Perhaps the difference is that O’Brien is still “hungry” and thus willing to bend over backwards – or, as Josh suggested:
After a lot of chaos and firings, Trump works his way to a non-entity who will simply allow him to do the job himself.
Or, as Bertrand suggests, this was Pompeo and Kushner leading Trump around like a trained animal. Just like everyone else.