O'Brien, best known for creating cult musical The Rocky Horror Show, is heading down the aisle after proposing to his German partner Sabrina Graf, who at 35 is half his age.
The 70-year-old star has known Graf for 11 years, and reveals he popped the question while appearing in a production of Oliver! last year.
He says: "We became closer and now I can't really breathe properly without her. I asked her to marry me in my dressing room when I was playing Fagin."
According to the Sunday Mirror newspaper, the couple will marry in an intimate ceremony in March at the Bay of Plenty in New Zealand, where O'Brien has lived since last year.
The father-of-three married his first wife, actress Kimi Wong, in 1971 and Jane Moss in 1983.