Assassin's Mark, Guardians of Empyrion, skill assignment and player buffs

[Ungoliath]

If i assign Assassin’s Mark on Guardians of Empyrion, doesnt it become worse because they wont benefit of Fighting Spirit’s OA buff? Wouldnt it be better to assign it to a player skill then?
Or how is this working?

Edit: Dam i just realized that just started 2 post with the same kind of problem.

Does Deadly Aim's % OA buff work on Guardians of Empyrion?

If my OA goes up because of Deadly Aim (Inquisitor), do Guardians of Empyrion (Oathkeeper, player scaling pet) get a higher OA too as long i got the buff up? Because i would only assign a devotion (Phoenix in this case) if they do get the higher OA.