OMGUS, or "Oh my god, you suck!" is the common term for a person voting another purely in retaliation for the latter voting the former. That is to say, it's when you vote for someone because they voted for you: "Oh my god, how dare you! You suck!"
OMGUS votes are annoying. They are not necessarily scummy, but they are worthless, since they are voting for a player for reasons other than their scumminess.
If a vote is made in retaliation, but it carries with it an actual case, then it's not necessarily an OMGUS vote. But it is often viewed with greater suspicion than if the vote and case were made before the other player's vote.
If you're town, the best way to avoid OMGUS voting is making sure all your votes have cases backing them. If people accuse you of OMGUS voting, take a step back, look at your case again, and ask yourself, "if the other player stops voting me, will I want to unvote as well?" If so, then you might be OMGUS voting.