Energy Department investigators now believe that the nation's 15 largest oil companies overcharged their customers by at least $5 billion from the start of the Arab oil embargo in 1973 through 1976.

The $5 billion figures represents new department projections of legal or administration actions planned against the oil companies before the end of the year.

The department's Office of Special Counsel has already alleged overcharges of $4 billion in more than 100 actions filed against the industry. Department sources said yesterday claims of at least another $1 billion in overcharges will be filed by year's end.

The department has already accused Exxon and Texaco of overcharges of more than $1 billion each.

The ongoing investigation involves violations of complex DOE pricing and allocations regulations.

Beginning in January, the DOE special counsel office expects to broaden that investigation to include the remaining 19 major oil companies and plans to look at overcharges since 1976. Sources at DOE say it is likely the overall investigation - which they hope to have completed by October 1980 - will result in even more allegations of overcharges.

About $130 million has been collected or earmarked for collection, from oil companies in the form of price rollbacks directly to consumers or penalty payments through consent decrees stemming from the probe, according to DOE special counsel Paul Bloom. The industry is challenging the remaining charges.

In general, the industry has said that the DOE regulations were confusing, that the companies interpreted them one way and that DOE is now saying the rules should have been interpreted another way.

The charges cover a broad area, including overstatement of costs, interpretive errors involving the regulations, pricing violations and others. The charges range from alleged elimination of proper discounts to a particular customer to widespread attempts by one company to overcharge all of its customers by disguising the origin of certain crude oil that should have been priced lower than it was.

Industry sources say Bloom is preparing several actions alleging that oil companies violated complicated "class of purchaser" regulations: overcharging customers by misclassifying them.The "class of purchaser" regulations allowed oil companies to charge different types of purchasers different prices for the same product.

Because the investigation is still under way, the amount of the alleged overcharges in particular cases has increased.

In one instance, for example, DOE charged in federal court last summer that Exxon had overcharged customers by $183 million through violations of pricing regulations. Less than a month ago, while amending its court complaint, DOE upped the amount to $500 million, based on further information gathered by DOE auditors.

The DOE special counsel's investigation has also led, in one instance, to successful criminal misdemeanor and felony indictments against three firms, including Conoco, for activity related to violation of DOE regulations.

Nearly 600 employes were assembled at the end of 1977 to create the Office of Special Counsel for the specific purpose of investigating violations of the highly complex regulations governing the oil industry.

In a related probe, Bloom's office has performed a special audit of allocation practices of the 15 largest oil companies from May 15 to July 15 of this year, "to see how they were behaving during the current crisis," he said. The results of that audit were forwarded to Bloom this week for study.

Bloom said yesterday that his office has been so preoccupied with attempting to meet strict deadlines for his investigations that he had to turn down an offer from his DOE superiors to add 150 people to the staff.

"It's really a commentary on how difficult it is to recruit under government procedures," Bloom said, "but we are so involved in completing our audits by the deadlines that have been set that we can't afford the enormous strain of going through the government hiring process." CAPTION: Picture, PAUL BLOOM...studying oil allocation practices