As I haven't read the light novel, it still very vague whether she is an antagonist or not. As in Nisemonogatari, Kaiki is an antagonist, but in the Second Season, he's become somewhat of a protagonist.
For now in Monogatari Series: Second Season, I believe she's antagonist from the fact that she lied and said that she is Meme Oshino's niece. (She pretend to be a good guy)
When she almost crashes into Nadeko, she said something like "I'm not supposed to meet you yet" and tells Nadeko to stop playing the victim. This implies that she's related to Nadeko becoming a snake god.
On very last episode when Kaiki gets attacked by someone. That someone mentions about Ougi, implying that she is the one behind Kaiki's attack. And when Kaiki about to pass out, he remember that name (Ougi), but he couldn't recall who is that. This means that Kaiki is telling the truth about Ougi not being Meme Oshino's niece.