Will an oil vinegar dressing spoil if left out of the refrigerator?

Eventually, yes. It will also spoil if left inside the refrigerator, it just takes longer.

Most vinaigrettes are made of nothing more than vinegar, oil and some herbs, all of which are shelf stable. The vinegar will never go bad, but herbs can loose their potency and oil will eventually go rancid. The onset of rancidity and the potency of the herbs can be maintained by keeping it refrigerated and the container sealed.