Otis R. Bowen, 67, a plain-spoken, low-keyed country doctor who was governor of Indiana twice and is described by friends as a middle-of-the-road Republican and strong foe of abortion-on-demand, will be nominated as secretary of health and human services, President Reagan announced yesterday.

Reagan said he picked Bowen to succeed Margaret M. Heckler, who is to be nominated as ambassador to Ireland, because he has qualifications "in excess" to head the huge department.

The HHS, which has more than 120,000 employes, has an annual budget of about $330 billion that includes Social Security, Medicare, Medicaid, the Public Health Service, the National Institutes of Health and the Food and Drug Administration.

Bowen is a physician who practiced family medicine for many years in Bremen, Ind., including his years in the state legislature from 1957 to 1972.

He has been on numerous scientific, medical and educational commissions and has written on health issues. He was governor from 1973 to 1981. In 1979, he served simultaneously as chairman of the National, the Republican and the Midwest governors' associations.

He is now a professor of family medicine and director of undergraduate family practice education at the University of Indiana School of Medicine.

The nomination requires Senate confirmation.

Two years ago, Bowen was chairman of the Social Security Advisory Council that focused on proposals to rescue Medicare from financial problems. The commission recommended major increases in tobacco and alcohol taxes to help fund Medicare, increases in premiums for Medicare beneficiaries, limits on annual payment increases to hospitals, and higher payments by patients for some services. It also improved benefits and set what amounted to a limit on how much a patient must pay if hospital costs mount to "catastrophic" levels.

"Doc" Bowen revealed himself at commission meetings as a plain-spoken, direct person whose medical knowledge enabled him to cut to the heart of issues and who would listen to all sides, then smoothly maneuver things in the direction he sought.

He has attributed part of his political success to being a family doctor. "You are closer to them than an ordinary friend," he said of his patients. "You see them through all sorts of difficulties, whether it be physical or emotional."

As governor, he said, he set out to "remove the burden of the cost of public education and local government from the property tax," shifting the revenue source to a sales tax.

Bowen's first wife died of cancer in 1981. He later revealed that he had administered DMSO, a drug without certification for some uses, to ease her pain. Associates said it was used under the direction of her doctor in an approved investigational drug-therapy program.

Bowen told reporters yesterday, "I did nothing illegal. DMSO is not illegal."

He has remarried and has four children, four stepchildren and 20 grandchildren and step-grandchildren.

Yesterday, 14 antiabortion organizations charged that Bowen is "pro-abortion" and favors euthanasia. In 1972, they said, he wrote a letter indicating that as governor he would sign a bill loosening the abortion law. They said he also favored "living wills," in which an individual directs that, if hopelesly ill, "heroic measures" need not be taken.

According to associates, Bowen's letter actually said the Indiana law should be loosened only "a very little bit" and should include strict safeguards against abuse.

They produced a letter dated last Oct. 22 to an antiabortion doctor, Dr. Paul Muller of Indianapolis, saying, "I have spent my life trying to save lives and not destroy them. I am opposed to abortion on demand but do not oppose abortion to save the life of the mother or in cases of rape or incest."