The Palestine Liberation Organization is more ready than ever to leave Lebanon, according to Lebanese and Palestinian leaders.
But to translate this readiness into action will require key concessions from Israel and the United States. These concessions are not minor.
They are understood to include an American promise to push for a final settlement of the Palestine problem and to negotiate with the PLO. Israel would have to withdraw from the Beirut environs and promise to withdraw from all of Lebanon soon after the PLO leaves the country.
The diplomatic plan that is emerging at long last is giving rise to hope in many quarters - pro -- and anti-PLO -- in west Beirut. One of the best signs is that after making a few rounds of discussions with political leaders one finds that this plan has not been undermined immediately - as has been the case with so many earlier peace and evacuation plans.
Leading Palestinian and Lebanese officials and political analysts have described the plan to the Monitor as follows:
* An international force (probably consisting of Greek, Canadian, French, and Italian troops) would be formed to supervise the departure of the 5,000 to 8,000 Palestinian fighters from west Beirut.
* Palestinians would reassemble in northern Lebanon and the Bekaa Valley, via the Beirut-Damascus highway, and travel thence to Syria. Some would stay in northern Lebanon (though without the consent of the Lebanese government or the rightist Phalange party), others would leave for Jordan, Egypt and Iraq.
* Simultaneously, Israeli forces would pull back from the outskirts of west Beirut, allow the international force to police the Palestinian exodus, and promise to depart from Lebanon in the near future.
''I believe that this is the plan now,'' says an editor with Beirut's leading independent newspaper, An Nahar. ''But it needs an Arab League covering and that is what is happening at Taif, (Saudi Arabia).''
Foreign ministers of Lebanon, Saudi Arabia, Syria, Kuwait, Algeria, and the PLO were meeting July 29 in Saudi Arabia to approve this plan, the editor said. He added that ''if the Saudis and the Syrians agree to it, then it is complete from the Arab side.''
That would leave the Israeli and American sides as the biggest questions. US special envoy Philip C. Habib was back in Beirut after meeting with Israeli Prime Minister Menachem Begin. Analysts here believe he may have an Israeli agreement to withdraw from the Beirut area in order to facilitate a supervised Palestinian evacuation. An American commitment to seek a solution to the Palestine problem and to talk with the PLO would be made, it is thought, only in secret and only if the PLO clearly states that it recognizes the existence of the state of Israel.
''There should be a pledge from the United States that if we leave we are only in transit to Palestine,'' PLO official spokesman Mahmoud Labadi told the Monitor.
Hani Hassan, chief political adviser to PLO Chairman Yasser Arafat, said Mr. Habib and Lebanese government leaders ''have no problem with the Palestinians any more. The problem is with the Israelis. . . . Habib has to tell the Lebanese (and then the Lebanese have to tell the Palestinians) if the Israelis are willing to leave Lebanon.''
Mr. Hassan, known as a PLO moderate, said he believes US-PLO dialogue can still be achieved. ''To win the recognition of the US is not a short process. It is a long process and we should continue it,'' he said.
Former Lebanese Prime Minister Saad Salam similarly said that the PLO is prepared to vacate Lebanon and that US-PLO talks should be a diplomatic goal. After discussing the countries of destination for the PLO with Saudi King Fahd and Egyptian President Hosni Mubarak by phone July 28, Mr. Salam said he saw progress ''in various directions.''
Mr. Salam said technical negotiations centered around ''simultaneous programming for the beginning of the departure of the PLO and the entering of the multinational force. He said the meeting in Saudi Arabia of Arab foreign ministers was ''to handle where the Palestinians will go.''
''The PLO is determined to leave,'' said Mr. Salam, who meets often with Mr. Arafat.
Mr. Salam criticized the Reagan administration for quickly dismissing the recent Arafat statement that the PLO accepts all United Nations resolutions, including those that call for Arab recognition of Israel (Resolutions 242 and 338). Instead, Mr. Salam said, the US should be nurturing such PLO statements, applauding them in the hopes of firmer PLO declarations in the future.
''Let them feel it will be well received,'' Mr. Salam says. ''Then they will give more. The Americans should know how determined the PLO is to make a breakthrough with them.''
Not surprisingly, Mr. Salam and other Lebanese and Palestinian officials see Israel as the biggest roadblock to a political settlement at this point.
Israeli military tactics over recent weeks have embittered these Lebanese. They believe Israel would prefer to bombard west Beirut forcing the PLO to leave under so much pressure that it will not be able to elicit any concessions from Israel or the US.