No, no it isn't. OMGUSing for a reason generally isn't a null tell. Just plain OMGUSing... ? That's a scumtell.
FoS Mr.Person for protecting Jim Groovester.
OMGUS isn't a scumtell because actions aren't really scumtells, it's thought patterns that are scumtells. Janus may have OMGUS'd, but did he really think Vector is scum? You can't tell based solely on the fact he OMGUS'd, you have to read into it. Here's an example for you to chew on. Murdering is bad, yes? Murderers are scum, right? What if I kill Hitler? Wrong action for the right reasons, would I still be scum? (DO NOT ANSWER THIS, PLEASE)
Of course, there are some actions that are kind of scumtells, but the thought process is again what really matters.
Suppose Org lurked. He knows it's bad, he should never do it. If he does, he's scum, but only because he's thinking he can get away with not doing things that he should be doing, but might incriminate him. For our example, rape would be scummy under all circumstances. It's not possible to justify. Which is why the real scumtell would be thinking rape is ok. Actually raping someone is unnecessary, just thinking it's ok to rape is proof enough that the person is scum.
Of course in mafia and real life, it's not easy to tell what a person is thinking, so actions are great clues. However, the actions themselves mean nothing, you have to read the thoughts behind the actions to derive any real meaning. So you tell me: Did Jim OMGUS Vector because he legitimately thought Vector was scum or to save his own ass? I think he thinks Vector is scum, so yeah, I'm going to defend him from unwarranted attacks. Of course, I dunno what Jim's thinking either, so you're going to have to ask him yourself if you want to prove me wrong.