Oil Windfall Acquisition Act of 1981 - Amends the Clayton Act to prohibit any major producer of crude oil and natural gas liquids from acquiring control or a majority of the assets of: (1) any other person in the business of energy exploration, extraction, production, or conversion, if such person has total assets of $250,000,000 or more and if the acquired majority of such person's assets totals such amount; or (2) any person not in such business if such person has total assets of $100,000,000 or more and if the acquired majority of such person's assets totals such amount. Declares that such prohibition shall not apply to: (1) an acquisition which would substantially enhance competition in domestic or foreign commerce or promote energy exploration, extraction, production, or conversion; or (2) the formation of joint undertakings for energy exploration, extraction, production, conversion, transportation, or distribution. Treats the acquisition of assets over three years as a single acquisition.

Directs a court, whenever a challenged acquisition has been consummated, to issue an order to ensure that the assets and operations of the parties to the acquisition are kept intact and held separate and that the parties do not interfere with or participate in the management or internal affairs of one another pending final adjudication. Specifies time limitations for actions brought by the Attorney General to enforce this Act.

Directs the Federal Trade Commission to report to Congress on the impact and effectiveness of this Act.