can u put a concealed squad in op fire and if you are in op fire mode can the unit move?

You can put a concealed squad into op fire either during the Place Units in Op Fire of the Command Phase or you can place them into Op Fire by spending an action in the Action Phase.

A unit in Op Fire cannot move.

ok… it says in the rulebook you must use all your actions up. what happens if you run out of units to use and only have units in op fire left?

Then you don't have any more actions. If all units are fatigued or in op fire then they have completed their actions. If you have units that have not fired or moved and you don't want to do either then you can just fatique them as an action.

If you have action cards in your HQ area you can buy those and use those as an action/actions, but this is not compulsory, you choose whether to use them or not, but once you have passed due to no actions left you cannot then come back after your opponent has done his three actions and use a card, once passed that is your turn finished.