The actor otherwise known as John Locke will play a Navy Seal commander who trained McGarrett (Alex O'Loughlin) and served with his father in Vietnam.
TVLine reports O'Quinn's character may hold some answers to solving the murders of McGarrett's mother and father.
No word on the exact amount of episodes O'Quinn will appear in.
In other casting news ...
• Scott Foley will guest star in the 'True Blood' Season 4 finale. Foley will play an old military buddy of Terry's and his role could become recurring if (when) HBO renews the series for Season 5. [TVLine]
• Griffin Dunne has joined the cast of Showtime's 'House of Lies.'The new comedy, which stars Don Cheadle and Kristen Bell, will see Dunne playing a famous consultant over a multi-episode arc. [Hollywood Reporter]
• 'Better With You' veteran Josh Cooke will have a recurring role on 'Dexter.' Cooke will play Louis Greene, a new lab intern at Miami Metro. [TVLine]