Dr. Thomas O’Brien's health had been failing for a long time prior to his death on Monday, according to the Boston Globe, while Ruth Reardon O’Brien passed peacefully on December 12.

Dr. O'Brien was an expert in antimicrobial drug resistance research -- serving as director of the Infectious Diseases Division at what is now Brigham and Women’s Hospital. He also taught at Harvard as an associate professor, finally retiring in 2019 at the age of 90.

O'Brien -- in an interview with the Globe after his father's death -- credited Thomas with inspiring his love of comedy ... remarking that the hardest he'd ever heard anyone laugh was when he went to see a Peter Sellers 'Pink Panther' flick with his dad.

Ruth started off as a law clerk on the Massachusetts Supreme Judicial Court before transitioning to real estate law -- becoming just the second female partner in her law firm's history in 1978.

Conan's not yet spoken about her death ... though we've reached out for comment.

Thomas and Ruth married in 1958 after Ruth's brothers introduced her to Thomas. They had six children -- including Conan -- and remained married for 66 years.

Thomas was 95, and Ruth was 92.