ONCE IN A WHILE things turn out absolutely right. That sums up our reaction to the news that the General Services Administration, after a nationwide design competition, has chosen architect Arthur Cotton Moore's bold plan for turning the Old Post Office into a lively complex of offices and shops. For one thing, Mr. Moore has had his eye on this building for some time. Along with the preservation group Don't Tear It Down, architect John Wiebenson, and Nancy Hanks and Bill Lacy at the National Endowment for the Arts, Mr. Moore has played a large role in persuading the government that the hulking Post Office is a community treasure, not an aesthetic disgrace.

But Mr. Moore won this competition on merit, not longevity in the field. And that speaks very well both for him and for the General Services Administration, where attitudes toward restoration and design have changed dramatically in the past few years. Holding a national competition was an innovative step in itself. Choosing Mr. Moore's plan, the least conventional of the three finalists, took the kind of audacity found all too seldom in government-land.

Mr. Moore doesn't just want to clean up the premises and invite the community in. His plan exploits the buildings eccentricities and charm, its neo-Romanesque tower and trimmings and especially its spectacular interior court. That startling space, 196 feet high, will become a grand public lobby full of shops, restaurants and liveliness, covered with a glass roof and ringed with balconies on the upper office floors. There will be a sculpture court, a Metro entrance and a tourmobile stop. There will also be energy-saving features such as rooftop solar collectors and office windows that actually open.

It all sounds quite glorious - and very right for a building that has been an appealing architectural oddity all of its life. The turreted clock tower and the cleaned-up granite facade are already landmarks on Pennsylvania Avenue. Now the whole structure can at last become a downtown centerplace for commerce, culture and public enjoyment, and a keystone of the entire Pennsylvania Avenue revitalization drive. That's quite a victory for vision and persistence, and a happy precedent for bringing life and light back into other durable public buildings everywhere.