The impasse over attendance of the Palistine Liberation Organization at next week's joint meeting of the International Monetary Fund and World Bank has been settled -- at least temporarily -- by "disinviting" all observer organizations.

The finace minister of Tanzania, Amir Jamal, who also is chairman of the joint session, announced that invitations -- which otherwise normally would be extended to a large group of financial organizations -- would not be sent this year.

However, two important groups -- the Organization for Economic Cooperation and Development and the Bank of Internal Settlements -- will be able to attend, in their usual observer status, the key policy sessions associated with the annual meetings, the sessions of the Interim Committee and of the Development Committee.

Meanwhile, Assistant Treasury Secretary C. Fred Bergsten said at a press conference that the United States is willing to go along with Jamal's decision in the interest of allowing the annual meeting to go forward with its urgent business. But he added that allowing the joint-meeting chairman (whose functions normally are honorary) to decide such questions as observer status "is not a sound basis on which to proceed."

There is still a posibility that the question of PLO admisssion will be brought to the floor, although American officials said privately that the "weighed votes" that could be lined up against such a move would be sure to defeat it.

According to some sources, Jamal came up with his disinvitation technique as a face-saver at the suggestion of the Saudi Arabians. Considering the influence of the Saudis in the Arab block at the IMF, this suggests that the issue may be allowed to fade away, at least for the time being.

Meanwhile, Bergsten said that nextweek's sessions will be "critical," coming as they do at a time of crisis in the world international situation. He called the economic ourlook for the world "somber" and said that the two Bretton Woods institutions "are in the front line" of the effort to help the world's poorer countries overcome their balance-of-payment problems.

He projected the real economic growth rate of the industrial world at no better than between 1 1/2 percent and 2 percent in 1981, up from one percent this year, while the less-developed world would have some slippage form a 5 percent rate in 1980.

It will be "a world of slow growth," Bersten said, and -- if not a "synchronous recession" -- it will "not be one of a rosy outlook." He sees the industrial world's inflation rate falling from 12 percent to 9 percent, while in the poor nations inflation might drop from 35 percent to 25 percent.

To cope with this grim outlook, Bersten said that the IMF and World Bank will have to play a larger role. He praised the INF for having taken major steps to adapt itself to the new and greater demands for aid by increasing access to its resources on less-onerous terms.

The American official also revealed that several additional steps will be proposed to the Interim Committee -- with U.S. approval -- for making the IMF loan policy more liberal. Chief among these is a plan by which a country can borrow up to 200 percent of its quota - that is, the amount it had deposited in its own currency -- for three successive years.

As Bergsten explained it, that would allow a country to borrow the hard-currency equivelent of 600 percent of its quota, a dramatic increase from the current limit of 150 percent.

Other new proposals to come before the Interim Committee will include an interest-rate subsidy for those countries that have been borrowing from a special IMF money pool called the Witteveen facility. Those tapping the Witteveen facility have had to pay market-related interest rates, which many of the poorer countries now cannot afford.

Neither Bersten nor IMF-Bank officials could say last night what impact the whole focus over the PLO would have on long-range plans both institutions have had to borrow from the Saudis and other oil-exporting nations with surpluses of funds. "We've had mixed signals on that," Bergsten said.

Obviously, everyone has fingers crossed, hoping that the situation will blow over. At the IMF, which has the most at stake immediately, officials have not given up.