User blog comment:KressKross/The President jokes.../@comment-10380701-20140725030046

As this takes place in Remnant, Ruby is not "quoting" anyone. In-universe, Ruby is just saying inspirational speeches from the viewpoint of the rest of Team RWBY. So only from a real life viewpoint does it seem to make less sense.

For "OMG", it's a common term. Even if you don't believe in Big G, you still kinda say it sometimes. It'll be jarring if they go "Jesus Christ!" though. But "Oh god" is still fairly believeable. A fictional world will always have a religion of sorts. It's actually weird if there's no religion at all.