COMES NOW THE star performance or the promise of it, the long-awaited testimony of Lt. Col. Oliver L. North, the Marine officer who. . . Who what? The startling thing about the weeks of Iran-contra hearings and the months of disclosures and the years of immersion in some of the events themselves is that so little is yet known of what now seems most important to know. It's not just what Oliver North did. Others have told of that, though his own account, as refracted through the terms of testimony that he was able to negotiate, is eagerly awaited. It's how this improbable middle-ranking officer, whose background suggested not much more than an aptitude for doing the brave and by-the-numbers things that Marines are expected to do well, could perform veritable prodigies of policy maneuver in Ronald Reagan's Washington.

To be sure, there is bound to be great interest in what Col. North has to say about what the president knew and when he knew it, about the whats, whens and wherefores of any higher authorization that the colonel had for his manifold activities undertaken in President Reagan's name, since all this bears on the political health of Mr. Reagan and on the authority with which he will be able to conduct the rest of his presidency. There will be close attention too to the range of deeds that led Col. North to bargain with the congressional committees for a grant of limited immunity from criminal prosecution.

Already it has been established that, to avoid the strictures that either his cabinet (in Iran) or Congress (in Nicaragua) placed on him, the president went to a back-door method of policy-making in which the determined and inventive Col. North played a central part. But how did he do it? Whatever happens to the man himself, this remains a town necessarily fascinated on at least two levels with the virtuosity of his deceit.

On the mechanical level, there is an abiding curiosity about the openings that the system offered to one inexperienced but remarkably fast learner, who found it possible to skirt routine practices, putative superiors and ostensible overseers in the name of serving the commander in chief. On the level of constitutional government, there are grounds for genuine alarm that the American government could be manipulated to allow an extended line of policy to be developed outside the system of checks and balances meant to inhibit secret executive power. This is the part of Oliver North's story that most needs to be understood now.