President Carter is considering tough new restrictions on mergers and acquistions by the nation's 23 largest oil companies in connection with his plan to decontrol oil prices.

Under an option sent him early last week as part of the decontrol package, any oil company within the top 100 companies in the U.S. (23 at present) would be prohibited from merging with or acquiring any other business worth $100 million or more, administration sources said.

Apart from any such antitrust restriction, which would require congressional action, Carter is certain to propose a windfall profits tax as part of his decontrol program.

Write House press secretary Jody Powell said yesterday that Carter has completed all the "major decisions" in his latest round of energy policymaking. The results probably will be disclosed in a nationwide prime time television address either tomorrow or Thursday.

The new antitrust limit on mergers and acquisitions is intended to soften liberal opposition to decontrol of oil prices, which could increase oil industry revenues by $13 billion or more.

First, it would head off any politically embarrassing deals - like the acquistion of Marcor, the parent corporation of Montgomery-Ward, By Mobil Oil in 1976-after decontrol. Prior to the Mobil-Marcor deal, many oil industry officials had stressed, as they are today, the industry's need for higher profits with which to finance new exploration and development.

Second, the restraint would effectively curtail acquisitions of other energy-related companies, such as Mobil's agreement last week to buy General Crude Oil Company for $800 million from International Paper.

Carter also is expected to propose tax credits for purchase of wood-burning stoves and furnances, in part to offset an expected political blacklash in New England, which is heavily dependent on oil as an energy source. Other tax credits may be included for investments in shale oil projects and solar energy installations.

In additional to the windfall profits tax, Carter may ask for what administration officials call an "OPEC tax," to capture future windfalls. If OPEC were to raise world oil prices unusually sharply at any point, much of the increase in U.S. oil prices would be taxed away directly. Officials were divided on whether a 40 percent rate or a 75 percent rate - favoured by presidential aide Stuart Eizenstat - was appropriate.

Sources also said that Carter has been mulling two decontrol options, both of which would have all oil free of controls by Sept. 30, 1981. The speed of the decontrol was the issue, with the difference in impact equal to 0.07 percentage point in the level of the consumer price index.

The slower, "back-loaded" option would allow the price of about one million barrels produced each day from "margina" wells - a sliding-scale definition linking output to depth of well and cost of production - to rise on June 1 from about $5.80 a barrel to$13. The remaining 2 million barrels a day of "lower-tier," $5.80 oil, and the roughly 3 million barrels a day $13, "upper-tier" oil would be decontrolled graudally over the next two and a half years.

The "front-loaded" option involves definiting away virtually all lower-tier oil by the end of 1980.

Because of fears of increasing inflation, the president probably will opt for the slower rate of decontrol, sources said.

The administration has in the past declined to endorse similar antitrust legislation proposed by Sen. Edward Kennedy (D-Mass.) that would cover all companies, not just those in the oil industry.