If a machine gun crew is has an officer present, can it still fire in op fire mode if pinned?

in short no,

the rally ability of the officer: A fresh , but pinned squad in the same hex as an officer may be activated with a concentrated attack action . the squad will fire at half firepower.

so on 2 acounts you can not

Fresh on page 8 says a unit a unit that has not been assigned an activation token eg fatigued or op fire is considered fresh, so an mg in op fire is not fresh

also the squad has been given an op fire action you can only give one action from the list of actions each action phase the rally rules say 'may be activated with a concentrated attack action' so you can not anyway as this is another type of action.