During the next three months the Department of Energy will file dozens of administrative actions accusing the 15 major oil companies of overcharging their customers by billions of dollars, department sources say.

The actions will come from the DOE's Office of Special Counsel, which has, for the past two years, been conducting an investigation of how the oil industry complied with the complex set of federal price and allocation regulations set up in the wake of the 1973 oil crisis.

That office, which must, under its mandate, complete its audit of the 15 largest oil companies by the end of the year, has already filed more than 110 such actions alleging just over $5 billion in overcharges. Although most of the actions still are being disputed in court or through administrative proceedings, companies have agreed to repay about $145 million.

But administration sources say the barrage of actions expected to be filed before the Dec. 31, 1979, deadline could bring the total of the alleged overcharges to $10 billion or more.

Because of the limited time involved, many of the audits of the major companies are coming to an end simultaneously, causing some concern at DOE that the rapid-fire actions in an election year will make them appear to be politically motivated.

There could be some days, officials predict, when as many as a dozen different actions alleging huge over-charges by several companies will be filed at the same time. Often, the officials say, the actions will deal with similar alleged violations.

Many of the actions already taken have been in the form of suits filed in federal courts, through the cooperation of the Justice Department. Others have been in the form of administrative enforcement actions seeking interest and penalties.

But DOE sources claim the remainder of the actions likely will be at least partly administrative in nature, because the Justice Department lacks the manpower to handle any more of the complex suits.

Energy Department officials have said that in most cases the violations have resulted from differing interpretations of the complicated system of regulations imposed in the mid 1970s, and that there is little or no proof that any of the firms deliberately violated the law.

The industry has said, in response to each of the charges when they were filed, that the government is changing its tune. Many of the interpretations they used, company officials say, were approved at the time by field officers of the Energy Department and its predecessor, the Federal Energy Administration. Now, they say, DOE unfairly has changed its mind and prefers different interpretations that would cost the industry billions of dollars.

The rampant confusion surrounding the regulations led to the creation of the FEA Task Force on Compliance and Enforcement, which in July 1977 recommended the creation of the Office of Special Counsel to investigate whether or not the industry complied with those regulations during the 1973-76 period.

The task force set the time limits, giving the office an extra year to complete the audits of the 20 smaller firms in the top 35.

Special Counsel Paul Bloom has said he will meet the deadlines, but added that his office has been strained somewhat by having to divert auditors for a special audit of the oil industry's allocation practices during this spring's gasoline crisis.

The results of that audit are expected to be made public soon, giving the first hints as to whether or not the major oil companies had a hand in the shortages that caused gas lines.

But it is the earlier audits that are resulting in the allegations of huge overcharges by the industry.

Last Jan. 5, for example, Bloom's office filed civil suits in district court against eight major refiners, alleging overcharges of about $1.2 billion. The suits charged the firms with multiple violations of regulations governing the calculation of costs associated with the production of natural gas liquids and natural gas liquid products.

And only five months later, DOE issued Proposed Remedial Orders -- the administrative equivalent of filing suit -- charging seven major refiners with pricing violations of $1.7 billion in connection with sales of domestic crude oil.

These and other actions have sought to rectify overcharges to such customers as local transit systems, independent marketers, retail gasoline dealers and consumers.

In settling some of the cases, the department has managed to secure millions of dollars of rebates to consumers through price reductions on future purchases.