O'Hare's character will make his first appearance early in the upcoming season and will stay on as a series regular.
A Tony winner for his role of Mason Marzac in "Take Me Out," O'Hare's acting credits also include the films "Duplicity," "The Proposal," "Milk" and "A Mighty Heart."
His TV resume includes appearances "CSI," the "Law & Order" franchise and a recurring role on ABC's "Brothers & Sisters" as Travis March.
Season 3 of "True Blood" is slated to bow in Summer 2010.
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