I am still playing my first two games but from i see the defenders have the advantage... Maybe that should be so...but a little feature that might balance this is to have individual team members with their own reaction times so that opp fire aside they would have to have an equal or higher reaction time to fire first.
I've never been a huge fan of the Laser Squad Opportunity Fire method. If anyone is old enough to remember Laser Squad's predecessor Rebelstar, that had a much fairer and more balanced method which I have always preferred.
Another fairly simple option would be to only allow Opps Fire to work if the Opportunist has a certain percentage more action points remaining than the player who is moving. That way if someone is waiting around a corner with Opps fire and the player comes bumbling past unsuspecting it then they will probably get blasted. But when you damn fine well KNOW there is someone waiting around the corner with Opps fire then it's pretty unfair that you currently can't (grenades aside) do anything about it. With this method though you simply save your action points up and ambush him on your next turn.