Almost from the moment Rep. David R. Obey (D-Wis.) was elected to the House eight years ago at the age of 30, it was clear he was going to leave a mark on the place.

What kind of a mark wasn't clear. "I think he probably got into a fight with George Mahon on his very first day," Rep. Lloyd Meeds (D-Wash.) said.

Mahon, chairman of the powerful Appropriations Committee on which Obey now serves, was just the first one Obey took on. He was appointed to a military subcommittee whose chairman made it a custom for members to stand when high-ranking generals and admirals appeared to testify. Obey refused. The chairman sent a staff member to inform Obey that he expected him to rise with the others. Obey told the chairman to go to hell.

In the House, a man off to that kind of start usually remains an outsider forever, playing the role of a dissident, a bomb thrower, almost always in a minority, and rarely effective.

Though Obey remains as intense, blunt and hot-tempered as the day he came, he is now an insider, with a growing reputation as a reformer and seats on not only Appropriations but the Budget Committee and the Democratic leadership group, the Steering and Policy Committee.

He avoided becomong a permanent outsider by a capacity for hard work, a perfectionist drive to be precise and not a shotgun attacker, a willingness to learn, and am obvious concern for the institution. Rep. Richard Bolling (D-Mo.), who learned from Sam Rayburn how to work on the inside, became Obey's mentor, teaching him to work with the leadership and use House procedures to get what he wanted.

Last week Obey insured his reputation for effectiveness as he became responsible for pushing through the House a comprehensive strengthening of the code of ethics, requiring fuller financial disclosure, an end to the use of campaign funds for personal or unofficial office expenses, an end to the use of campaign funds for personal or unofficial office expenses, and end to "slush funds" of private contributions for a member's unofficial expense account, and a 15 per cent limit on outside earnings.

Since June, Obey has headed first a task force and now a commission of House members and outsiders that is providing a broad overhaul of internal operations - from the scheduling of House sessions to the accounting system.

Though the ethics package passed by an overwhelming 402 to 22, that vote belied the fight it took to get it passed. In the end it was a motherhood issue the members were scared to oppose, but behind the scenes hard fights took place.

Last June, after Wayne Hays was toppled when it was revealed he had put his mistress on his payroll, Obey was appointed by then Speaker Carl Albert to clean up the mess. Not the sex scandal mess - that was up to individuals - but the mess Hays had left by using the chairmanship of the House Administration Committee to shower the members with perks through the accounting system.

By July Obey had drafted a package that required members to account for almost every penny spent on office expenses, travel and newsletters. He ended a practice of allowing them to take the stationery allowance in cash and using it for anything from putting kids through college to buying a car.

Obey acknowledges he thought financial disclosure was enough. But he was convinced by commission hearings and by Rep. Lee Hamilton (D-Ind.) that outside income was the source of an enormous potential for conflict of interest.

Obey was also convinced by his close friend, Sen. Gaylord Nelson (D-Wis.), Nelson took Obey to see Senate Majority Leader Robert Byrd (D-W.Va.). Byrd agreed the Senate must adopt a similar ethics code, appointed Nelson to head the committee and the two worked on the details of the package informally together.

Obey's first indication that the code he was producing was in trouble came when he began to take it to committees that had jurisdiction over its various parts.

First the ethics committee made a prohibition on gifts so restrictive that a member would have to account for every desk calender sent to him.

Then the House Administration Committee balked at its part.

Obey flew into one of his rages. Sources said he marched in to Speaker Thomas A. (Tip) O'Neill, who had endorsed his plan for a new ethics code, and threatened to abandon the project unless O'Neill got the committees in line.

O'Neill called the House Administration Committee members and that afternoon they adopted the package Obey wanted.

The Rules Committee became the biggest problem.

Obey wanted a ruling that would allow floor votes up or down on each proposal but would prevent members from amending it to death. The five committee Republicans were committed to opposing that rule. More importantly, at least four Democrats on the 16-man committee had personal problems with the income limit.

Claude Pepper (Fla.) and Morgan Murphy (Ill.) had lucrative law practices. Shirley Chisolm (N.Y.) was a star on the lecture circuit. And B. F. Sisk (Calif.) opposed the limit.

O'Neill invited the Rules Committee Democrats to breakfast, at which he pounded the table demanding their cooperation.They were not convinced. Next day, O'Neill got tougher. They were all his personal friends, O'Neill said, but by God they would find themselves on the District of Columbia Committee next year if they failed him on this one.

Every Rules Committee democrat fell in line. "He's got the guts to do what Speakers are supposed to do - lead," Obey said.

Adoption of the rule was the key vote on the floor. The whip system went to work and it passed 266-153, almost a straight party line vote. "Once you had that rule it was very hard to attack each issue frontally," Obey said.

"Obey thinks he's the goddamned Pope," one member stiff chafing over the income limit said. "He thinks once he proposes something, it's got the doctrine of infallibility attached to it."

"Obey's a tough and gutsy guy," said Meeds, a member of the commission. He acknowledged Obey's style may someday cause a backlash, but said, "His great strength is he's so honest and aboveboard and committed, he is forgiven a lot."

It's not that Obey is a protester of the '60s or even an angry young man from the '50s. It's just that he's always been that way. In seventh grade he decked a nun who hit him. In high school he became politicized when right-wing backers of Joseph McCarthy attacked his history teacher.

Obey got his master's degree from the University of Wisconsin in 1960, intending to become a political science professor. But he thought experience in the state legislature might help. At 24 he was elected from his home county in Wausau. To make money he tended bar at his father's supper club on weekends. "That's where I really learned about politics. Members of the county board came in for lunch and after meetings, and I relly learned how county government operated."

Obey is not finished with his work. He still intends to go over the management system of the House, do something about audits of accounts and the support systems before his commission goes out of business in December.

He feels that the Republican are attacking Congress as a institution and dragging it down. "It's unfair as hell and politically stupid," he said.

He does not believe that his work will be hailed in the press or will do much for the image of Congress. But he himself is satisfied.