House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said yesterday that he opposes raising the Social Security retirement age as a way of solving the system's long-range financing problems.

O'Neill and President Reagan have endorsed a Social Security rescue plan, worked out by a bipartisan presidential commission, that would eliminate the systems's short-term deficit and about two-thirds of its long-range deficit.

To close the long-range gap, many Republicans have suggested raising the normal retirement age from 65 to 66 in the next century, while Democrats generally have favored raising the Social Security tax.

"We think bascially it's wrong to increase the [retirement] age," O'Neill said yesterday. "Of course, the Democrats haven't signed off on it yet. We would have a problem with that one, and I am personally opposed to it."

O'Neill's comments came as the National Association of Manufacturers and the huge Save Our Security (SOS) coalition endorsed the rescue plan in testimony before the House Social Security subcommittee.

Meanwhile, the American Association of Retired Persons (AARP), the Gray Panthers, several business groups and three federal employe unions voiced strong opposition to major parts of it.

The American Federation of Government Employes (AFGE), Service Employes International Union and National Treasury Employes Union said the rescue plan's provision to extend mandatory Social Security coverage to newly hired federal employes would undermine the Civil Service trust fund, do little to help Social Security's financial situation and threaten overall pension levels for future federal employes.

Kenneth Blaylock, president of the AFGE and of the AFL-CIO public employe department, also argued that the immediate cash flow to Social Security from coverage of new federal employes has been enormously overestimated.

Former secretary of the old health, education and welfare department Arthur S. Flemming--speaking for nearly 140 SOS affiliates representing the aged, the disabled, labor unions and social welfare and church groups--said SOS endorsed the rescue plan but opposes any rise in the retirement age.

The NAM, while endorsing the bipartisan rescue plan, favored the age-66 concept. Also testifying in favor of the plan were former Social Security commissioner Stanford G. Ross and Milton S. Gwirtzman, who headed an earlier advisory commission on Social Security problems.

The AARP and Gray Panthers, which belong to the SOS coalition but refused to go along with its position on the package, said the bipartisan plan's provisions for delaying this year's cost-of-living increase and taxing benefits were too harsh. The AARP also opposed Social Security payroll tax increases recommended in the rescue plan.

AARP executive director Cyril F. Brickfield said Social Security's short-term revenue needs should be met by increasing various taxes temporarily. He said the best approach to long-term deficits is to encourage later retirement and penalize early retirement.

The National Association of Realtors favored giving the self-employed a 25 percent tax credit--instead of a deduction--for their Social Security tax payments, which the rescue plan recommended raising. The National Federation of Independent Business criticized the rescue plan's tax increases, particularly the payroll tax changes. The National Association of Wholesaler-Distributors also criticized the payroll tax changes and the proposal to tax some Social Security benefits.