Around 1995, O'Quinn made guest appearances in "The X-Files" and "Harsh Realm", produced by Chris Carter, who also cast him in the film "The X-Files: Fight The Future" and then once again in the final season. In 1996 O'Quinn started acting in the television series "Millennium" as Peter Watts, also produced by Chris Carter. O'Quinn held this role for all three seasons of the series. O'Quinn holds the distinction of having played four different characters within the extended "X-Files"/"Millennium" continuum (the two shows being classed together since both Lance Henriksen's character of Frank Black and Charles Nelson Reilly's character of Jose Chung have appeared in both shows).