OPEC'S AGREEMENT on oil prices represents the success of a strategy that Saudi Arabia has been pursuing implacably for nearly three years. It has been an awesome demonstration of economic power. Now the question is what the Saudis will choose to do next.

The struggle within OPEC has been between the Saudis and a group of governments--Iran and the Africans--who were the leaders in the push for larger and faster oil price increases. To force the high-pricers to conform to Saudi price policy, the Saudis have been selling their oil a little below other countries', and they have produced a little more of it than customers wanted. They have drowned their competitors in Saudi oil.

That's what created the present slight surplus of oil on world markets--the alleged glut. Although that glut is genuine, the thing to remember is that the Saudis alone created it, and they can eliminate it whenever they please. The Saudis have hinted that, after imposing a unified OPEC price, they would take the responsibility for seeing that prices don't decline further. That means a little tightening to end the present downward trend. Do they mean to maintain the price in dollars? Or will they maintain it in real terms--that is, dollars adjusted for inflation? Don't count on the market's staying soft much longer.

But the Saudis' exercise of oil power has implications beyond pricing. It has created an interesting opportunity for them, in cooperation with the United States and the European governments, to deal with Libya. Now is the time for the United States to pull all American citizens out of Libya and, with the Europeans, stop buying Libya's oil. There will be no oil shortage.

Nigeria was one of the high pricers and, because of it, has lost most of its customers. From last December to August, its production fell by two- thirds. If Libya's present customers swung to Nigeria, the Nigerians could supply them immediately with plenty of capacity to spare. Nigeria is a democracy, with a large population and urgent needs for development capital. Unlike Libya, Nigeria does not use its oil revenues to run assassination squads, or to invade its weaker neighbors, or to instigate subversion in other countries.

With the AWACS vote, the Senate supported President Reagan in a policy of--everyone hopes-- working with Saudi Arabia toward greater stability and peace in that region. Libya is a major threat to that stability and peace because of the enormous oil revenues that it commands. The United States and Saudi Arabia are now in a position, with a little European assistance, to turn off the Libyans' oil money.