guys with this guy can I chose my jaws as melee weapon and buff myself? Or I have to chose only oldblood melee weapon and not mount?
I'm hardly an expert, but I think you can use Ancient Warlord any of the Melee Weapons listed on the model's own Warscroll. The reasons I say this are, first, a "Saurus Oldblood on Carnosaur is a single model" and, second, the keywords on the scroll include "Saurus" and "Hero," satisfying the conditions of the Command Ability as I read it.
Digging a little deeper, it seems to me that any and all units with the "Saurus" and "Hero" keywords on their Warscrolls that are within 20" of the Saurus Oldblood on Carnosaur when it uses this Command Ability benefit from it, is that true? The wording says "whenever a Saurus Hero from your army" satisfies that range requirement it benefits from the bonus. The question I have is whether the Hero (or Heroes, if I'm right) can be buffed with Ancient Warlord in the Hero Phase (that's when Command Abilities go off, right?) and then move outside the range during the Movement or Charge Phases and still enjoy the bonus, or whether, instead, the wording on the Warscroll requires them to still be in range during the Combat Phase to get the +2 to their Attacks characteristic. Does that make sense?
Yes. It affects the Oldblood as well because units are always in range of themselves (clarified by FAQ). I quickly glanced over the rule text and I am rather sure the distance is measured when the attacks happen. So no, they shouldn't move away. EDIT: 20" is plenty though.