If there is one sure sign of an agency's growing power, it is the creation of a private "watchdog" organization whose sole purpose is to figure out what the agency is up to.

So let it be known that the Office of Management and Budget has officially arrived. In a small office at 13th and L Streets NW, just outside the fringes of the K Street power lobbying corridor and the DuPont Circle haven of the liberal activists, lives OMB Watch.

For most of President Reagan's first term, OMB led the way as the administration twisted, turned and tightened the rest of the federal government. In a backhanded tribute to OMB's increasingly pivotal role, more and more private organizations that owe their existence -- at least in part -- to Uncle Sam are turning to OMB Watch to find out what Uncle Sam is up to.

Sixteen months ago, in the summer of 1983, OMB Watch was a mere project, started by a coalition of anxiety-ridden nonprofit groups concerned about an OMB proposal to ensure that government-funded organizations did not lobby with government funds.

A melting pot of 1960s and 1970s activists, the groups had names like Center for Community Change, Interchange Resource Center and National Community Action Foundation. They feared that OMB was trying to harass them out of existence or cut their lines of communication with Congress.

That controversy has died down temporarily with the publication of a watered-down lobbying rule, but OMB Watch lives on with a $200,000 budget, four foundation grants, a staff of four and a membership estimated at 1,000 organizations. Local groups pay $35 annually; their national organizations and government agencies (such as Larimer County, Colo.) pay $250.

The organization is staffed by Gary Bass, technical coordinator Shannon Ferguson, and attorney David Plocher. The academic credentials of Bass and Plocher are duly noted on the group's letterhead ("Gary Bass, Ph.D.," and "David Plocher, J.D."); what Ferguson lacks in academic lettering he makes up with a handlebar mustache of heroic proportions. Their resumes include having worked for such organizations as Wayne State University and the Human Services Information Center (Bass) and for such issues as the antipoverty crusade (Ferguson) or education of the handicapped (Plocher.)

"You're going to do an article on OMB Watch?" said one official acquainted with the group. "It's nothing more than three guys and a goddamn mimeograph machine . . . . They don't have ideas. They just have a vague set of ideological principles and a mimeograph machine."

"A comment on OMB Watch?" said OMB general counsel Michael Horowitz, the person most responsible for the lobbying regulations. "Zero. Zero. I'm not going to dignify them with a comment."

OMB Watch is the quintessential offspring of the kind of organizational inbreeding that is endemic to Washington, where meeting planners meet to plan meetings. It was born into a world where "network" is a verb and "community" is an all-purpose modifier. Its members are national federations of local groups. In effect, OMB Watch is an advocates' advocates' advocate, monitoring government for government monitoring groups.

It also is likely to flourish in the coming months because its existence is based largely on the fears Reagan's OMB generates among liberal activist groups, people who participated in the war on poverty, the civil rights movement and other political crusades.

"We had a strong feeling that OMB is increasingly powerful. All of us have had experience with being in government or some aspect of it . . . . OMB is such a powerful agency and so well hidden," Bass said.

Still principally known for its role in hacking away at federal agency budgets, OMB in the past four years has consolidated new authority, including the power to review and often veto most regulations and the power to control what information the government can gather and, to some extent, how it can use that information. See story, Page A4.

Other policy decisions -- such as redefining the official poverty line written into many welfare laws and programs -- fall at least partially under OMB's purview. "That's an incredibly hot issue to our community," Bass said.

The group's membership was bolstered after Bass set off on what he described as a $25,000 "dog-and-pony show" to 34 states last year. And any time the group is quoted in a major newspaper, he said, it attracts new members -- and money.

"There certainly is a growing respect for the expertise OMB Watch has generated, and for its understanding of grants management," said Julee Kryder-Coe, the associate director of Independent Sector, which represents such nonprofit groups as the Boy Scouts of America and the American Heart Association and was one of the groups that helped form OMB Watch.

It was the lobbying activities of organizations receiving such funds that OMB's controversial lobbying regulation cost control rule sought to address.

Independent Sector is not a member of OMB Watch now, she said, but, "we look to them" for advice on any question about the appropriate use of federal grant money.

"If not 100 percent, at least 90 percent of our members are federal grantees," Bass said. To pry information from behind OMB's walls, he said, "We operate as a network. Community groups feed us the information they're hearing and we check it out. Our network also includes concerned professionals who are civil servants within OMB."

What is the point of it all, working so many steps removed from the groups -- and the human beings -- whose interests they are supposed to be furthering?

Said Bass: "It's the same work we were trying to do before. This is the way we sustain ourselves. We still have commitments. We're finding different ways to achieve them."