(written from a Production point of view)
William O'Connell (born 20 August 1933; age 85) was the actor who played Thelev in TOS: "Journey to Babel". He has made no other appearances on Star Trek, but has acted in many other television projects as well as in several films, many of which feature other Star Trek alumni. He filmed his scenes on Friday 22 September 1967 and Monday 25 September 1967 at Desilu Stage 9.
O'Connell made his screen acting debut with an uncredited appearance in the 1961 film 20,000 Eyes, co-starring fellow TOS guest actor Rex Holman. The following year, he appeared in the Thriller episode "A Wig for Miss Devore" and the Twilight Zone episode "Cavender Is Coming", both featuring John Fiedler, another TOS guest actor. O'Connell would go on to co-star with Fiedler in a 1975 episode of Kolchak: The Night Stalker. He went on to appear in a 1963 episode of The Lieutenant, a series written by Star Trek creator Gene Roddenberry and starring Gary Lockwood. Also in 1963, both O'Connell and TOS star James Doohan made uncredited appearances the 1963 comedy The Wheeler Dealers.
In 1964, he acted with Roy Jenson in an episode of The Outer Limits and with Morgan Woodward in an episode of Rawhide, and in 1966, he appeared with Meg Wyllie in the "Holloway's Daughter" episode of Bob Hope Presents the Chrysler Theater. He then co-starred with DS9 guest star Brian Keith in the film Way... Way Out. O'Connell would again appear with Keith in 1971's Scandelous John, which also featured Richard Hale and Bill Zuckert. In 1968, O'Connell appeared in John Sturges' Ice Station Zebra, as did TOS guest actors Lloyd Haynes and Jonathan Lippe.
In 1969, he appeared with Perry Lopez and Robert Pine in the Wild Wild West episode "The Night of the Pistol", with Jean Simmons in the drama The Happy Ending, and with Robert Easton, Harve Presnell, and Ray Walston in the Clint Eastwood Western Paint Your Wagon. He would go on to appear in two more Eastwood westerns: 1973's High Plains Drifter, co-starring Marianna Hill; and 1976's The Outlaw Josey Wales, with Erik Holland. He also appeared with Eastwood in the 1978 comedy Every Which Way But Loose, also featuring Roy Jenson, and its 1980 sequel, Any Which Way You Can, also with Roy Jenson as well as George Murdock and Logan Ramsey.
1971 saw O'Connell co-star with Peter Brocco, Christopher Shea and Garry Walberg in the Odd Couple episode "A Taste of Money". And in 1974, he appeared opposite his TOS co-star William Shatner in Big Bad Mama, an exploitation action film also featuring Dick Miller and Noble Willingham.
O'Connell's last known on-screen acting appearance was in the 1991 made-for-TV movie The Haunted. Also starring in this movie was George D. Wallace.