References: Traveller Main Rulebook; Book 1: Mercenary
If an enemy near to a character takes a significant or minor action which does not specifically target the character (such as attacking another enemy or attempting to run past) the character may get an attack against them. The opponent must be in personal range. The character may react immediately after the action(s) have been carried out and make a melee attack or grab. It costs a reaction to make the attack and this will affect initiative scores and apply -1DM penalties to skill checks as usual.
An enemy who attempts to get up from a prone position will also grant an opportunity attack.
Recoil and Heft
Alteration to Recoil and Heft, Pg. 61, Traveller Main Rulebook
Instead of an initiative penalty, the penalties for recoil higher than Strength DM will be applied to a player’s next action (possibly forcing a check on a simple action) UNLESS the player expends a minor action to steady themselves after firing/before the next action.
Alteration to Reactions, Pg. 61, Traveller Main Rulebook
Reactions, including dodging, parrying and opportunity attacks, just impose a -1 DM to all checks until the following round.
Alteration to Suppression Fire, Page 72, Book 1: Mercenary
Suppression fire inflicts a negative DM on the skill checks of a target equal to the effect of the successful attack. Suppression fire gets a +2 to ‘hit’, replacing the -2 listed.