The Open became an official PGA Tour event in 1995.
The change is best understood through the PGA Tour's treatment of official money and victories. The Open had long been a central championship on the European Tour, which was formed in 1972, but in 1995 prize money from The Open was included on the PGA Tour's official money list for the first time. Earlier PGA Tour seasons were subsequently adjusted to include the championship in official money and win statistics.
This administrative recognition helped make the tournament more attractive to American players, because their performances counted directly toward PGA Tour records and earnings. The Open remained an international championship organised by The R&A, rather than becoming an ordinary U.S.-based tour event.
The wording can cause confusion because The Open was already recognised as one of golf's four major championships. The important date is 1995: that is when its prize money and results entered the PGA Tour's official statistical framework. John Daly won that year's championship at the Old Course at St Andrews, defeating Costantino Rocca in a playoff.