I can't find the disadvantages of putting unit into Op Fire.

I'm guessing that a squad placed in Op Fire at the end of action phase will not be allowed to move during next action phase.

Could someone clarify   how and when you should place them in Op Fire or Fatigue them.

Cannon Fodder said:

A unit in op fire can react to movement of your opponent's units. However, it cannot move nor take a concentrated fire action against any of your opponent's units that does not move!

You are correct; this also means that when you do so during the status phase, the above also applies.

Normally putting a unit in op fire is always a better choice than simply fatigueing it. Note that when a unit uses op fire it is fatigued as well! The importnat exception here being that when only machine guns use op fire, they are not fatigued and stay on op fire. However, the same machine gun may not fire at the same enemy unit twice nor may a unit be fired at more than once in the same hex (unless combined fire is used, which technically isn't firing more than once, actually, but anyway…)

Hope this celars it up!