Some online sources list seven people as having been in the opening-night cast who were not in the cast list in the opening-week playbill. Five of these were replacements fairly early in the run: Remo Arlotta, Joseph Cunneff, June Graham, Pat Meany and Arthur Ulisse.
Another, Jack Harwood, was listed in the pre-Broadway New Haven and Boston programs, but he was cut from the show before Broadway. In his book Oklahoma! The Making of an American Musical, Tim Carter is uncertain whether Harwood made it to opening night in New Haven.
The seventh is Gary Smith, Jr. We have yet to find him listed in any playbill for the show.
Another oddity of the cast lists on these sites is the inclusion of seven "swings." There were no swings listed in the opening-week Broadway playbill. We have not come across playbills from any point in the run that list swings. Six of the seven performers listed as swings on those sites were in the opening-week playbills, but listed as regular members of the cast, not swings. The seventh, Pat Meany, was (as already mentioned) a replacement during the run.
Lastly, someone who was listed in the opening-week playbill's cast list, Kenneth LeRoy, is not listed on these sites as having been a member of the original cast but is listed among the replacements.