House Speaker Thomas P. O'Neill Jr. (D-Mass.) said yesterday, "I favor draft registration," but he added in the same breath that he doesn't see actual draft calls resuming.

He told reporters that draft registration is necessary to prepare the nation "for the essentials. One of our great faults is that we're not ready for the essentials."

O'Neill's endorsement comes as the House is nearing a vote on a proposal to require men who become 18 after Dec. 31, 1980 - which would be after the November elections - to register with their draft boards.

The draft registration requirement was added by the House Armed Services Committee to a bill authorizing the Pentagon to obligate $42.1 billion in fiscal 1980 for ships, aircraft and other weaponry. The Senate has decided to vote on draft registration in a separate measure.

The House Rules Committee heard arguments yesterday that registration is too important and controversial to be tacked on to a weapons bill. Witnesses at a hearing urged the Rules Committee to follow the Senate's lead by putting registration in a separate bill.

Draft registration would "breathe new life into the dormant Selective Service System," Rep. Bob Carr (D-Mich.) told the Rules Committee. He said it would "bring back draft cards, the draft lottery" without any full congressional review of the conscription machinery.

"We just don't think it's fair," Carr said. He chided the Armed Services Committee for putting off the registration requirement until after next year's elections, declaring: "If it's an important thing to do, do it now."

Carr and Reps. Ronald V. Dellums (D-Calif.), Patricia Schroder (D-Colo.) and Charles H. Wilson (D-Calif.) were on the losing end of a 35-to-4 vote by the Armed Services Committee last month to require 18-year-old men to register.

"Registration convincingly shores up a major existing deficiency in our mobilization capability," argued the committee majority in its report on the weapons bills. Neither President Carter nor Defense Secretary Harold Brown has endorsed registration. The Joint Chiefs of Staff have done so.

The House Rules Committee did not vote yesterday on whether to send the weapons bill, including the registration provision, to the floor. Chairman Richard Bolling (D-Mo.), who was absent because of a back ailment, had asked that a decision on the measure be postponed until his return because he considered registration such an important issue.

Veteran House staff members doubt that the Rules Committee will send the measure back to Armed Services for a rewrite. The effort to strip the registration provision from the weapons bills most likely will be made on the House floor, probably after the July 4 congressional recess, they said.

The requirement for young men to register with their draft boards was suspended by President Nixon in 1975. The House Armed Services Committee provision would require men who become 18 on Jan. 1, 1981, and afterward to register under procedures to be spelled out by the president.

Opponents contend registration is the first step back toward the draft, which ended in 1973. Backers of registration are considering offering an amendment on the House floor of reinstitute a limited draft. This, it is being argued, would give lawmakers the chance to vote against the draft and for registration, dramatizing the difference between the two issues for their constituents.