The Department of Energy has reached a $25 million settlement with Union Oil Co. on charges that the company violated oil pricing regualtions during 1973 to 1981.

Energy department and oil company officials are expected to sign the settlement agreement on Monday.

The case against Union Oil Co. is part of an estimated $4.7 billion that DOE might be able to collect in overcharge cases. Recently the Reagan administration has been castigated for what critics say is a listlessness in pursuing those cases. The Union settlement, the fourth since Reagan took office, may help dispel some of the criticism.

A tentative settlement with Union was agreed to in January 1981. Since then processing the settlement within DOE and getting the Justic Department to sign off on dropping litigation against Union have delayed final agreement.

The settlment requires Union to distribute $17 million to commerical customers, including transportation companies, utilities and others. Under the agreement, another $8 million will be returned to Union's retail dealers based on gasoline purchases. In addition Union will pay $50,000 in lieu of civil fines.

In a hearing earlier this year, DOE officials said that Union was anxious to settle and that the process was moving forward. "I just wonder how much longer we can take these assurances of vigorous enforcement efforts seriously when the actions seem to speak louder than words," subcommittee chairman Rep. Toby Moffett (D-Conn.) said then. Yesterday Moffett said he was pleased to learn of the settlement.

Earlier this week a DOE official told another subcommittee that recent reorganizations in the department had reduced DOE's ability to collect in overcharge cases. A General Accounting Office study reached similar conclusions.