[1] O'Hara completed officers' training at Maxwell Air Force Base Alabama at the rank of second lieutenant.

In May 1959, O'Hara was assigned to Patrick Air Force Base in Cape Canaveral, Florida, in support of the Mercury Program.

Her job was to set up the Aeromed Lab, the crew sleeping quarters, the exam area for the astronauts in Hangar S, and be with them as their nurse.

Before every launch she performed the pre-flight physicals including height, weight, temperatures, blood pressure measurements.

[5] In 1964, she moved to Houston to set up the Flight Medicine Clinic at the Johnson Space Center.

In Houston she performed all the routine check ups and more detailed exams, along with all of the administrative tasks, and taking care of the astronaut families.

Between 1971 and 1974 O'Hara was the manager of the Medical Operations Division in the Flight Medicine Branch until moving to Ames Research Center.

[2] She was involved in the decision to ground astronaut Ken Mattingly from Apollo 13 due to the concern he might have contracted German measles.