The American dollar has bounced back in the past six months to regain some of its old zest on Tokyo's money market.

Many experts, including those at the Bank of Japan, think it will slide downward again soon against the yen, but for the moment the dollar is the healthiest it has been in nearly a year.

Much of this has been due to the dollar defense program launched by the Carter administration last fall, analysts agree. It is also partly due to the perverse effects on currency markets of the worldwide price increase in oil and to a decline this spring in Japan's trade surplus.

Since last November, when the Carter adminstration acted, the dollar has increased by about 26 percent in value against the yen, rising early this week to the level of 225 yen.

It was the highest mark in nearly a year and produced a sudden turn-about in Japanese reactions. A year ago, the Bank of Japan was buying up dollars to stem the long decline. This week the bank was furiously selling dollars to protect the yen.

A cheaper yen-and its mirror image, a stronger dollar-make it more difficult for Japan to keep its promise to reduce its huge trade surplus. The cheaper yen makes Japanese goods more attractive abroad, and at the some time, makes imports more costly.

American bankers interviewed here today said they anticipate another turn to a lower dollar with most of them expecting it to settle at a level of about 200 yen to the dollar.

"It's overvalued now," said one trader for an American bank. "I can see it at about 200 by the end of the year."

That is in line with predictions in Washington this week by Prime Minister Masayoshi Ohira, who said he considers the yen undervalued now and that he expects to see the exchange rate stabilize eventually at about 200.

Their prophecies seemed to be in part self-fulfilling, because the dollar fell sharply when the Tokyo exchange opened this morning for the first time since Ohira's comment was published. The dollar fell by three yen, a sizable decline, in the opening moments of trading.

Until last fall, the dollar had been falling steadily since January 1977, pushed downward for the most part by the huge continuing Japanese trade surplus. Even at its latest high-water mark of 225.80, the dollar was worth about 22 percent less than in the early days of 1977.

The turnaround began last November when Carter announced a package of measures to defend the dollar, which at times was as low as 186. The measures included a large fund to intervene in the world's foreign exchange markets. The dollar shot up by nearly 10 yen overnight in Tokyo and has continued a zig-zag rise ever since.

A further reinforcement came with the announcement in March that the Organization of Petroleum Exporting Countries would raise world oil prices by 9 percent.

That meant that all currencies of oil-buying nations would be affected, but is had an especially powerful impact on the yen. The impact on Japan was relatively greater than other countries because it imports virtually all of the oil it uses.It was forced to spend a lot more dollars for the same amount of oil and the effect was to push the dollar up against the yen.

It also means that in the long run Japan's manufactured goods should become proportionately more expensive than those in other countries because so much of their cost reflects oil prices.

The dollar also improved in recent months because Japan's large trade surplus with the United States showed signs of declining. Huge surpluses in past years were the principal reason for the dollar's fall against the yen.

When Japan closed out its fiscal year on March 31, its balance of payments surplus was $12 billion, about $2 billion less than in the previous year. Moreover, the current account surplus in March was only about $600 million, about one-fourth what it had been a year earlier.

A major reason for the changing trade balance is a notable increase in Japanese imports in recent months, particularly of manufactured goods from Europe and the United States.

Opinion is divided on whether that trend will continue in the favor of the United States. A number of experts have speculated recently that the import binge will taper off quickly, pointing out that it has reflected more than anything else the availability of cheap dollars through most of last year. They made American imports less expensive and more competitive in Japan.

The Bank of Japan seems determined to drive the dollar back down to about 200 as quickly as possible. The comments by Prime Minster Ohira and Teiichiro Norinaga, gover-of the bank, were seen as efforts to "talk" it down and the bank began intervening heavily in the Tokyo foreign exchange market on Wednesday when the dollar soared through the 225-yen mark for the first time in 11 months.

The bank sold millions of dollars to take the pressure off the yen and it closed today at 220.80. CAPTION: Picture, Ohira lunches with Chase Manhattan Bank chairman David Rockefeller. UPI