Ole Ernst graduated from Odense Teater in 1967 where shortly afterwards he was hired as a stage-actor. Ernst had his debut playing Chino in Westside Story (1965). In 1969 he jumped from Odense Teater to Det Kongelige Teater where he continued stage-acting until 1976. Ernst performed in a lot of classic stage-plays such as Jeppe På Bjerget, Erasmus Montanus, Hamlet and Amadeus.

In 1973, Ernst played the lead role in Hans Kristensen's gangster-movie Flugten which was followed by the two sequels Per (1975) and Blind Makker (1976). The three films became very popular, and are to this day still considered some of the best Danish films ever.

Kristensen's Per-trilogy launched Ole Ernst's career as one of the finest actors of his generation. A short list of other great films Ernst starred in: Hærværk (1977), Terror (1977), Mord I Mørket (1986), Peter von Scholten (1987), Epidemic (1987), Sort Høst (1993), Okay (2002) and Rembrandt (2004).

In 1994, Ole Ernst was re-hired by Det Kongelige Teater.