MOST OF THE OIL companies have now published their earnings for the past year, showing -- as perhaps you expected -- a brisk improvement in earnings from last year. Or any other year, for that matter. But the companies emphasize that their fouth-quarter earnings were in general just about the same as in the fourth quarter of 1979. The suggestion is that the profit curve is flattening out. Maybe so. But you ought to keep in mind that, since last March, the oil industry has been paying the windfall tax -- a tax that it has repeatedly described as onerous, damaging and excessive. Even after paying this heavy new tax, the oil industry was making as much money in late 1980 as it did before that tax was imposed.

Many people in the oil industry are pressing the Reagan administration to repeal the windfall tax. But events since it was enacted last winter have fully confirmed the wisdom of it. The purpose of this tax is not to punish or harass the oil industry -- which, we shall state once again, is doing a valuable job very much in the national interest and doing it with great technical skill. The industry argues that, without the tax, it would have more money to invest in exploration for more oil. But the enormous rise in oil prices is already providing the industry with incentives beyond the most manic fantasies of the wildest wildcatters. Drilling is up 30 percent over this time last year. The ending of price controls promises still greater incentives and threatens still more oil-field inflation as companies bid against each other for scarce equipment and technicians. Exploration could hardly be proceeding faster.

The case for the oil windfall tax is simply that it provides an essential shock-absorber to the American economy, during a vast and sudden transfer of income to the oil industry from everybody else. It limits the rate at which money piles up in the oil companies' cash registers. This year the windfall tax will probably bring in about $19 billion. That's not a small figure. By comparison, the total revenue of all American corporation income taxes is running around $65 billion a year. The truly staggering thing is that the oil companies are paying a tax as large as this one without suffering any decline in net income.

The proper use of the windfall tax is to recycle oil profits back to the rest of American business. The oil companies bitterly object to this concept, with its implication that they are winning their high profits at the expense of other businesses. But something very much like that is clearly happening. The oil people argue that economic growth is not a zero-sum game. Certainly not -- but there is a limit to the speed with which revenue flows can be wrenched toward one sector, away from all the others.

The best reason for a cut in business taxes this year is precisely to offset some of the tremendous weight of energy costs on other industries. A very rapid shift of income to the oil producers can only result in inflation and unemployment elsewhere in the economy. The windfall tax on oil, coupled with the coming reductions in other business taxes, is indispensable to limit the strain on the national economy as oil prices keep rising.