OPEC doesn't feel like the right analogy. OPEC is a foreign state. Arrakis seems to be, like all planets, owned by the galactic empire. It is by several orders of magnitude the most important planet. A better analogy would be if, say, Hawaii, contained the bulk of the world's energy resources. It is strange to me that the galactic empire doesn't rule this planet directly with extreme security measures. Why isn't the empire immediately taking charge directly when there's any sign of conflict on the most important place in the empire?
Why not? You'd have to ask Frank Herbert for a definitive answer, but the next best is to read the books. There are many factions involved, the emperor is not actually in a position of absolute authority. The Great Houses, the Spacing Guild, Bene Geseret, Bene Tleilax and others also wield influence. There are checks and balances in place (by custom, by law, by force), and one of those is that the Emperor does not control Arrakis directly. It would put him in a position with too much power that the others (envious or desirous of semi-selfrule) would object to, probably violently. Arrakis is also a source of great wealth, and by giving it out to specific Houses, the Emperor is granting a favor (at least on the surface) and buying their allegiance.
The Emperor, ostensibly, controls all the planets. The political system is based on fiefdoms in the book. He hands the administration of Arrakis to a particular noble based on his own political objectives and trust.
They are essentially nukes, yes. But melange (the spice) is only available from Arrakis (in the books it's discussed that they try and find ways to make it elsewhere, but without spoilers I can't so much more than that it doesn't work). If the OPEC nations were your only or principal source of oil in this world, and you nuked them, what immediate impact would you expect from that? A global economic shutdown, and almost certainly being ostracized (if not worse) from the international community. Setting off nukes and making it practically impossible to export melange from Arrakis would be similarly effective at collapsing the galactic empire and economy.