In a free market "scarcity with a low price" is a contradiction. If there's scarcity the prices will be high, not low. And nobody's going to be reducing production if the prices are high.
> instability of oil production that OPEC tries to prevent
First of all, if the goal is to prevent instability OPEC is doing a terrible job. Secondly, a cartel is not needed to prevent price instability, as demonstrated by the hundreds of other commodity markets around the world which are not controlled by cartels engaging in price fixing schemes.
As with any cartel, the purpose of OPEC is to maximize profits for its members, artificially fixing the price of oil at a level higher than what it would otherwise be in a free market not controlled by a cartel. Price stability is a side effect of that, not the goal.