The United States has obtained "virtually everything we've asked for" in negotiations to open the Japanese telecommunications market, Commerce Undersecretary Lionel Olmer said today.

Olmer, the head of the U.S. trade delegation, said many areas of the $25 billion-a-year market have not been touched by the talks and further long-term negotiations are needed. However, he said that in the areas that have been discussed, the United States has done well.

Olmer's remarks were among the most positive assessments of the negotiations from the U.S. side. And they come at a time when the Reagan administration is anxious to defuse threats by Congress to limit Japanese imports into the United States.

Other U.S. trade negotiators have taken a far less positive view of the talks, which since late last year have been the focus of trade tensions between the United States and Japan.

Olmer gave his assessment of the trade talks after the United States and Japan completed a four-hour session here in separate negotiations over sales of foreign electronics in Japan. That field and telecommunications are among four sectors that the two sides agreed in January to discuss in an effort to reduce the U.S. trade deficit with Japan, which totaled $37 billion last year.

After today's session, Japanese officials characterized the talks as fruitful. Americans, however, said the Japanese offered few new steps of substance and insisted on many points that the U.S. position was based on misunderstanding.

The telecommunications talks have so far centered on regulations for computer networks and "interconnect" equipment, such as switchboards, facsimile machines and telephones.

"The Japanese have done virtually everything we have asked them to do in the telecommunications system short of giving us a blank check for goods not yet received," Olmer said.

Olmer said pressure from President Reagan and Congress and the cooperation of Japanese Prime Minister Yasuhiro Nakasone had helped bring about this success.

Skeptics within the U.S. government and business community argue that the Japanese have left themselves considerable leeway to keep old practices alive in substance if not in name.

In an April 24 letter given to Moriya Koyama, vice minister for posts and telecommunications, Olmer listed three general areas for the next phase of talks:

* Confirmation of details of changes that the two sides have worked out only in broad principle.

* Changes in laws that govern use of the radio waves in Japan. The United States has said the current law makes it difficult for American firms to sell radio-based equipment such as mobile telephones.

* Access to Japan's market for U.S. firms providing international phone service. At present, the Japanese company Kokusai Denshin Denwa (KDD) is alone in the field.

In the electronics talks today, the two sides worked with a nine-point list submitted by the United States earlier. It covered highly technical disputes in such fields as customs clearance, patent registration and access to Japanese research and development programs.

Japan presented a detailed proposal, expanding on an earlier one, for mutual reductions of tariffs in some 40 electronics items, such as computers and magnetic tape. U.S. officials have promised to study the proposal but said they tend to believe that a mutual cut would not necessarily reduce the U.S. trade deficit with Japan.

One of the few points of specific new progress, according to U.S. accounts, was a Japanese promise to grant foreign exchange licenses in one day for certain types of imports. At present, it can take up to three weeks.

In addition, U.S. negotiators raised three new electronics issues:

* Cooperative efforts to rein in plans for new factory capacity in the semiconductor market.

* Increased procurement by Japan of foreign electronics.

* Removal of barriers to foreign electronics imports thrown up by Japan's close-knit industrial groups, which often favor companies inside the group when selecting partners.

The United States also has asked that Japan consider changing its "Buy Japan" policy for satellites devoted to uses other than telecommunications, such as broadcasting and mapping. Japan promised to give an answer next month.