I am not sure it has anything to do with ability to hold the ball. Defensively it is obvious the difference between the two. Oscar is really discipline in term of doing his marking duty where as Mata is quite poor . Offensively they are somewhat different. Oscar is like mezut Ozil. He does not always want the ball. He will move to the wing to create space for our winger to cut inside. Simply put, tactically he is brilliant both Offensively and defensively. Mata on the other is typical Spanish player. He wants the ball on his feet and will move into space to receive the ball. In term of technical and playmaking skill, it is not even close, he is by far our best playmaker. Oscar is definitely better suited for Mourinho type of football.