As for the changes in his appearance, SVTFOE is pretty general with their art style; different animators and storyboarders will put their own spins on things, and even then, designs will evolve and change as the show progresses! Pretty cool stuff!

I actually think this wasn't so much "Oskar being weird", and more "we're finally getting to know Oskar a bit more". Also, he dropped out of school and seems to be making his own way in the world - chances are something like that is bound to change someone, even if only a little bit. He may be more outspoken and independent now, and I'd be really interested to see if they're planning on bringing him in to more episodes in the future!