THE OIL WINDFALL tax, in the butchered form in which the Senate has now passed it, is outrageously weak. It makes enormous and dangerous concessions to an industry that is already very large and already enjoys more than normal prosperity. The defense of this bill is that the tax must be kept light to provide the industry with incentives. But the incentives in this bill are wildly and disproportiontely high. The House version of this tax is in every espect superior.

The crucial issue is the tax on newly discovered oil. A year ago, under price controls, the oil sold for $12.60 a barrel. Today it's been decontrolled, for compelling reasons of national interest, and it goes at prices around $30 a barrel. Over the next two years, as the price controls are lifted on all oil, the revenues of the oil producers will more than double. The purpose of the windfall tax is to return some of that enormous flow to the public and -- beyond the obvious matters of equity -- provide a shock absorber to the rest of the economy against this immense diversion of money to one industry.

Under the Senate's bill, the windfall tax on that $30 barrel of newly discovered oil would be 95 cents.The windfall is real. The tax is a joke.

If the price rose to $40 -- not, unfortunately, beyond imagination -- the tax would be a terrific $1.80. The House bill is a different matter. Its top rate is 60 percent on this oil, compared with the Senate's 10 percent. On a $30 barrel of oil, the tax would be $6.90. Is that really unenduraably harsh and confiscatory? Remember that it would leave the producer $23.10 for oil that, produced a year earlier, would have gone for that $12.60. Producers' revenues will rise rapidly under either bill. The question is how fast to let them rise.

The Senate bill, its authors say, would raise $178 billion over the 10 years. That sounds impressive. But the figure is computed on price and production assumption that show new revenues to the oil producers -- income, that is to say, in addition to their present income -- of perhaps $750 billion over those 10 years. That aggregated 10-year figure obscures an important aspect of this tax: in the Senate's form, it would rapidly decline over the coming decade. The heaviest rates are on the oil from the reserves in production longest, and they are rapidly being depleted.

In a particularly offensive gesture, the Senate has exempted what it calls the small independent producer from paying any windfall tax at all. The small independent producer is an individual selling no more than, 1,000 barrels a day -- which, with decontrol, will mean an income of $10 million a year. How's that for populism?

Now the two versions of the windfall tax will go to a House-Senate conference. Since President Carter wants it enacted quickly, perhaps the White House will be tempted to push for the usual split-the-difference compromise. But both the president and the House would do better to disregard the calendar and stand fast. The House bill sets no more than a modest tax. The case for it becomes more compelling with every surge upward in oil prices.