Defenders Ion Ansotegi and Inigo Martinez struck with headers either side of halftime at the Anoeta before Osasuna centre back Jordan Loties was shown a straight red card for tripping Carlos Vela when he was through on goal.
Forward Antoine Griezmann nodded a third in the 56th minute, Gonzalo Castro finished off a sweeping break in the 82nd and substitute Haris Seferovic helped himself to a goal two minutes from time as the visitors crumbled.
San Sebastian-based Sociedad have 17 points, two behind sixth-placed Getafe who host Valencia on Sunday, while Pamplona club Osasuna are mired in 16th spot, a point above the relegation places.
Promoted Almeria's 1-0 victory at home to Real Valladolid, the south coast club's second win of the campaign, lifted them onto nine points.
- Sports & Recreation
- Inigo Martinez
- Carlos Vela
- Antoine Griezmann
- Jordan Loties
- Gonzalo Castro
- Haris Seferovic