Harold J. I'Brien, the outcall massage king, is not about to let a little thing like The District of Columbia.

Luckily, his studio apartment above the Beauty Fair at 735 11th St. NW is just a brisk stroll from Superior Court and likewise the Tearoom at Woodward & Lothrop's, where O'Brien is accustomed to have lunch is in easy walking distance of the courtroom where he went on trial yesterday and figures to spend much of the next three weeks.

And since Judge John G. Penn promises to adjourn by 5:30 p.m. each afternoon, O'Brien should have no trouble getting to his office in the Dupont Circle Building for what he says are the peak hours in the massage business - 6 p.m. to 2 a.m.

Through his firm of Swingers International, which advertises "outside massage . . . radio dispatched" in the yellow pages. O'Brien arranges to send women - whom he describes as independent contractors rather than employees - to designated hotel rooms across the city. These women carry telephone beeper units, according to O'Brien, so he can contact them whether or not they have a phone handy.

The case that leaves defendant O'Brien so unconcerned is an important one to the D.C. Corporation Counsel's Office. Important enough to require as many as 150 witnesses, three prosecutors, and a massive array of documents and physical exhibits.

"They have got every investigator I have ever seen," complained defense attorney Stuart Stiller yesterday. Stiller went on to describe himself and his client as "just two little citizens trying to make our way."

The witness rolls include Ralph Nader, City Councilman David Clarke, White House assistant Mary E. King, five representatives of the D.C. Department of Finance and Revenue, two reporters from The Washington Post and one handwriting expert.

Through them, the prosecution promises to establish that O'Brien, as proprietor of a photocopying service between October, 1971, and June, 1975, collected sales taxes from scores of customers but failed to turn over a penny to the D.C. government.

In his opening statement to the jury, Assistant Corporation Counsel Kenny Pells employed a rubber-tipped schoolteacher's pointer and a large easel-borne display of the key elements of the government's case.

Defense attorney Stiller then proceeded to concede virtually all of Pells' allegations. His client had been in business, had collected taxes, had neglected to file or pay, said Stiller, "but the most important element . . . is, was it 'willful.'"

Stiller and the prosecution agreed that O'Brien had been granted an extension of all tax dealines in the case to Jan. 19, 1976. So the question, said Stiller is "did he willfully fail to file and pay on Jan. 19, 1976. . . You will see that it wasn't willful . . . he couldn't pay on that date." Stiller did not specify why O'Brien couldn't pay.

The charges against O'Brien, 30 counts in all, are misdemeanors that each carry penalties of up to one year's imprisonment and a $5,000 fine.

Prosecutors and tax investigators have admitted, however, that they have no idea how to collect fines or taxes from O'Brien. "He lives like a monk," said one assistant corporation counsel, who concluded that O'Brien must have ammassed "a fortune" through his various enterprises - chiefly the massage business, but also dating and escort services, an auto driveway firm, and photocopying.

During a lunch recess, O'Brien discussed ways in which someone can do business, not pay taxes, and get away with it. He owns no property, he said - "there's nothing you can't lease" - and he talked about the inadvisability of putting one's money in a bank account, where it might be subject to seizure. A better policy, said O'Brien, would be to convert all income into traveler's checks, purchased under an assumed name, and store them in a safe deposit box "in Canada. . . or somewhere."

O'Brien said his business activities are all legal, and described himself as a "workaholic."

He insists that his massage customers be at least 30 years old, he added, "because I remember what I was like before I was 30. . . not mature."