The interior scenes inside of the hunting lodge were all filmed on a soundstage at Pinewood Studios in England on a life-sized set. The opening scenes at Kenny's house and the exterior scenes on the island were all filmed on location in Spain. Additional "highway" scenes were probably filmed in Northern Michigan by a 2nd unit crew.
The screenplay was adapted by author David D. Osborn from his own novel of the same name. The movie credits refer to a work called "The All-Americans" but in actuality the book was simply called "Open Season" (Dial Press, New York, 1974).
Veteran Spanish actor Simón Andreu is credited with the role of "Barman" in the cast listing during the credits of all three of the English-language versions, but his performance not actually present in any of those prints in the versions which have turned up on home video.
The trivia item below may give away important plot points.
There are three different endings for the film, depending upon which version you find: The British print of "Recon Game" and the American Columbia Pictures print of "Open Season" both conclude with a still frame image of William Holden as he walks through the gate at Ken's house leading Petey by the hand. The "Recon Game" version also uses slightly different music and ends more abruptly than the Columbia print of "Open Season". But the somewhat longer widescreen Scandinavian release titled "Jaktoffer" concludes with a dialog scene after Holden walks through the gate and effectively surrenders to the police, with a final shot of the street in front of Ken's home and a reprise of the movie's theme song as the credits roll.