What are you talking about? OPEC is not a labour union, which is clearly what’s being discussed here. You may as well say a horse is a duck because they both shit.
OPEC works the same way as a union. They are both groups of competitors that band together to form a cartel, using similar tactics to increase the price of the product the sell. In one case, oil, in the other, labor.
OPEC is made up of oil producing countries and their combined output. They are extremely similar except that one deals in labor, and the other in petrochemicals.
One is big enough to influence the global market price for it's product, the other is small and localized and has effectively no influence on the going market rate for it's product.
On a local level? Maybe? But consider that the US workers of the most valuable car company in the world (by a factor of about 3)are not part of the UAW, so it's debatable at best.