THE SEVEN-NATION summit opening in Ottaw on Monday is being billed as something of a command performance in the international center ring for Mr. Reagan. This is excessive. Ottawa is the latest of a series of summits of the major industrialized democracies that began in 1975. These meetings have not resolved the inevitable frictions among the participants, but they have at once expressed and enhanced their sense of common purpose. Mr. Reagan's aim need not be to star or to transform the series but to get in the flow.

Not so long ago the Europeans were supporting a tight-money, low-employment consensus and complaining of inflation's being generated by the United States. Now, with a baby boom's wave of job-seekers washing over the labor market, that consensus has broken down entirely in France, is in difficulty in Germany and is under fantastic pressure in Britain. Now, too, the Europeans, who tend to have a short memory in these things, are complaining of the tight-money, low-employment policies that the Reagan administration is employing to fight inflation at home.

It would be nice if the United States and its allies were not on a seesaw. But they are. That leaves Mr. Reagan to explain that, sympathetic as he is to the allies' woes, he is determined to stick to a policy that has a chance of helping Europe as well as the United States in the end. He could be excused for thinking that in their currently besieged state the Europeans find it easier to agree on American responsibility for their predicament than on anything else.

Mr. Reagan would have his hands full if he had to deal just with the fact that the Ottawa seven's economic cycles are out of phase with one another. But he also must deal with a painful disharmony in respect to the Soviet Union. The other six have economies or constituencies tipping whatever government is in power more toward detente than the government of the United States. Nowhere is this more in evidence than in Germany, the key American ally and a country in which currents of opposition to things nuclear, military and American are swelling at a rate that the administration can't continue to downplay without peril.

With his hard-to-get posture on nuclear negotiations with the Kremlin, Mr. Reagan is testing the outer reaches of the allies' fidelity to American leadership. If the alliance were not under heavy economic strain, he might get away with it. With the economic crisis, it becomes a close question. He could bust the alliance.