Mahmud Zahar, one of the main leaders of the resistance group, said on Monday that disagreements over the PLO charter and the size of Hamas' representation in the council had delayed its incorporation.
"Our decision to participate in the PLO is not new. It is a longstanding and clear decision," Zahar said after a meeting with Palestinian President Mahmud Abbas in the Gaza City.
The Hamas leader said some progress has been made in minimising disagreements.
"A committee grouping the general secretaries of the Palestinian factions was set up to look at this issue and the results have opened the way to the entry of Hamas and other factions into the PLO," he said.
Zahar said Hamas' entry did not mean that it had renounced its commitment to a Palestine incorporating modern-day Israel.
The PLO charter calls for the creation of a Palestinian state only in land conquered by Israel in 1967.
"A committee grouping the general secretaries of the Palestinian factions was set up to look at this issue and the results have opened the way to the entry of Hamas and other factions into the PLO"
Mahmud Zahar, Hamas leader
"We are renouncing not one iota of Palestinian land but we can reach a temporary compromise with other factions," he said.
Speaker of the PLO's parliament Salim Zaanun said the PLO executive committee would meet on Tuesday for talks with representatives of Hamas and Islamic Jihad, which is also outside the organisation.
Fatah is the largest movement in the PLO, which represents Palestinians living in the occupied territories as well as the diaspora.
The PLO controls the Palestinian Authority set up by the 1993 Oslo accords with Israel.
In a related development, Palestinian President Mahmud Abbas' Fatah faction, in a major step towards democratic reform, is to hold primaries to choose its candidates for the July Palestinian parliamentary election, officials in the group said on Monday.
The 120-member Fatah Revolutionary Council, the faction's second most important decision-making body, agreed on the move at the end of a two-day meeting, the officials said.
Meanwhile, an explosion ripped through a car late on Monday in Gaza City, wounding a lawyer who was in the vehicle, as well as two bystanders.
The lawyer, identified as Majad Jafarwi, was seriously hurt.
Police closed off the area after ambulances raced to the scene.
Palestinians said Jafarwi, 55, is well known and is not politically involved.