A massive coalition of more than 100 national organizations is getting ready to battle President Reagan if he proposes major Social Security cuts. r

The group, representing millions of aged, retired and disabled persons, is headed by Wilbur J. Cohen, former secretary of Health, Education and Welfare under President Lyndon B. Johnson and one of the fathers of the Social Security program.

Called SOS for "Save Our Security," the group was formed two years ago to fight the Carter administration when it proposed a series of Social Security cutbacks. With Cohen and others tramping the halls of Congress, it helped stave off all the Carter proposals except reduction of disability insurance benefits.

"We will reactivate the organization if they propose cuts," said Cohen in a telephone interview from the Lyndon B. Johnson School at the University of Texas, where he is now a professor.

Cohen said individuals or groups associated with the coalition over the past two years included Robert Ball, former commissioner of Social Security; ex-House Ways and Means Chairman Wilbur Mills (D-Ark.), United Auto Workers president Douglas Fraser, AFL-CIO chief Lane Kirkland, the National Conference of Catholic Charities, American Association of Retired Persons, National Council of Senior Citizens, National Black Caucus on Aging, Paralyzed Veterans of America, Disabled American Veterans, and many others -- about 115 organizations all told.

He said he has been working since the election to seek additional organizations. Other sources said funding is being sought to hire a full-time staff if needed.

Cohen said he expects to have things in shape for a big lobbying effort within 30 to 60 days, when the Reagan administration program becomes more clear.

Both he and Ball said that they personally oppose suggestions to raise the retirement age for full benefits from 65 to 68 in gradual steps and so do many of the organizations in the coalition.

Jim Hacking, legislative chief of the AARP, said his group would work with the coalition to oppose slashes in benefits for persons already retired or reasonably close to retirement, but is more open to long-range structural changes to strengthen the financial position of the system than others in the coalition.

The giant Social Security system will pay more than $159 billion in fiscal 1982 to nearly 40 million retired and disabled persons and their dependents, plus $57 billion for medicare. But the system is running into short term financing problems for the next few years because the cost of living and the rate of unemployment are rising faster than expected in 1977, when new taxes were voted to put the system on a sound basis.

Many critics believe Social Security taxes are already as high as they should go ($1,975 this year for the worker earning $29,700 or more). Among cost-cutting proposals by a Reagan transition study team were granting lower benefits to future retirees than scheduled under present law, raising the retirement age to 68 and granting cost-of-living increases that don't fully keep up with inflation.

Others at various times have proposed cost-saving devices such as reducing double-dipping by federal employes who, in some cases, are eligible for both Civil Service and Social Security retirement benefits; ending benefits at age 18 for the child of a dead or disabled worker instead of allowing them to continue to 22 if the child is in school; and eliminating the $255 burial benefit.

Many of these proposals are opposed by the bulk of members in the SOS coalition. However, several said that they wouldn't object to using a different cost of living index, as proposed by the Carter administration last week, to govern automatic annual cost-of-living increases, if they believed it was genuinely a more accurate measure -- even if it meant lower increases.