If a squad chooses to fire and then move, is the squad not subject to Opportunity Fire until after it fires (when actually moves) of does the act of firing (after announing a fire and move option) initiate Opportunity Fire?

In my opinion, the unit doesn't expose itself to Opp Fire until it moves.

To my knowledge Op Fire is activated when a target -moves-. So firing does not activate Op Fire.

Another good question. According to page 19 of the base game rules, Op Fire is available only after a unit has moved into a new hex. "Immediately after the active unit moves into any hex, before it may do anything else, it is subject of Op Fire attacks by enemy unity in Op Fire mode."

Also note that exiting an entrenchment/pillbox not only costs a movement point (an often forgotten rule!) but triggers Op Fire too! So it doesn't have to enter any other hex to trigger, as one may guess. Same with squads leaving their transports.

Just pointing that out

A squard is only subject to OP-fire when expending MP to enter a hex. It does not have to be a New hex though. Exiting a vehicle and entering the hex, or leaving a entrenchment both enters the hex the squad is already located within.