No. If the Attorney General disagrees with the OLC then the DOJ's position is that of the Attorney General. The OLC INFORMS the Attorney General's opinion, it does not override it. What the AG says, goes.
As for if it's legal or not, that's not really accurate either. It currently has at least three injunctions against it in various jurisdictions. At best you could say its legality is contested.
That's too easy & too true. The President's only recourse in such situations is to fire the AG, which Trump has done.