WARSAW, Poland (AP) _ The traditional oath in which Poland's legislators once promised to do everything for its ''socialist development'' became a relic of history Tuesday as deputies took a new pledge to ''uphold the legal order.''

The Council of State, Poland's collective presidency, passed a decree Saturday authorizing the new oath and eliminating a reference to building socialism, which the Solidarity-led opposition considered unacceptable.

Legislators elected in June to the Sejm, or parliament, and the new Senate met Tuesday for the first time for the swearing-in ceremonies.

The opposition had no quarrel with the first part of the oath in which legislators pledged to ''honestly and thoroughly fufill duties toward the nation.''

But many opposition members felt they would be unable to take an oath which would have required them to do everything to ''strengthen independence and sovereignty, for the successful socialist development of the Polish People's Republic.''

Zbigniew Rudnicki, a deputy from the communist-allied Democratic Party, was named the senior-speaker of the Sejm. At Tuesday's session, Rudnicki called the name of each deputy and asked them to swear to work for Poland's good. All 460 members of the Sejm took the oath.