There's a little problem with President Carter's fiscal 1981 budget:

It seems that somebody forgot to account for rising oil prices.

The $615.8 billion spending plan the president sent Congress two weeks ago is based on last September's crude oil prices of $24 a barrel.

But oil prices are topping $30 a barrel these days, and analysts say they're likely to rise still further between now and the end of fiscal 1981.

That could make a big difference -- between $1 billion and $4 billion in the defense budget alone budget experts say.

As a result, the $15.8 billion budget deficit the president proposed two weeks ago could end up near $20 billion. That is, unless oil prices fall.

The low $24-a-barrel estimate doesn't spring from any gusher of optimism among White House planners.

When Senate conservatives were demanding that the president support higher defense spending last summer, Carter needed a set of cost estimates.

In making the calculations, planners reached for the handiest energy-price assumptions available. They took the ten-current cost -- $24 a barrel.

Carter announced his new defense targets. The senators proclaimed themselves more or less happy. Only then, they say at the budget office, did anyone realize the oil-price estimates would be too low by 1981.

But there's a problem in changing figures once a president has adopted them as his own. To admit a mistake then would have been politically embarrassing.

There also were other considerations. Some experts weren't convinced that the oil price increases occurring then would be permanent.

Others didn't want the government to seem to be sanctioning further increases. And some were reluctant to bloat the deficit. The oil-price estimates stayed intact.

What to do about the oil-price goof has stirred a dispute within administration policymaking circles.

Fred. P. Wacker, the Pentagon's chief budget officer, told Congress last week that the extra fuel costs could total $3 billion to $4 billion a year in fiscal 1980 and 1981.

But James T. McIntrye, the director of the Office of Management and Budget, asserts that the increased cost will be "substantially less than you have heard."

He's pledged that the administration will have it all figured out properly by early spring.

Meanwhile, the Pentagon is exploring "alternative" ways to account for the extra money, say by cutting back on fuel consumption or trimming other programs back.