Would a Rogue get their Sneak Attack bonus on an opportunity attack? Would it make a difference if they already applied their sneak attack during the current round of combat? In this case the Rogue is a Swashbuckler and so he doesn't need to have advantage or an ally within 5 feet of the enemy to apply sneak bonus. But to me if someone is running away from you then they are well aware you are there and you shouldn't be able to sneak attack them.  

Let's have a look at the text.

You also gain an additional way to use your Sneak Attack; you don't need advantage on the attack roll to use your Sneak Attack against a creature if you are within 5 feet of it, no other creatures are within 5 feet of you, and you don't have disadvantage on the attack roll. All the other rules for Sneak Attack still apply to you.

So it looks like as long as those conditions are met, it does not matter if the enemy is aware of you or not. I would say this feature of the swashbuckler has less to do with the enemy being aware of the swashbuckler as it does with the swashbuckler's ability to get a precise shot in as the enemy attempts to retreat.

To complement what TexasDevin already correctly explained, the Reaction Sneak Attack is actually something that most Rogues should look for, as the Sneak Attack feature clearly states "Once per turn", not "Once per combat round", therefore once the character's turn is over, a Reaction attack can still benefit from Sneak Attack, since it is not the character's turn anymore.

A "once per turn" ability can be used more than once in a round if for instance the character has a feature that gives more than one turn in a round. I know there is at least one such ability as a higher level feature somewhere in the PHB.

They can proc sneak attack on opportunity attacks. It is also why every rouge should take Sentinel.