The NBA Cup is the competition’s annual in-season tournament, with all matches except the final also counting towards the regular season standings.
The Thunder were beaten in last year’s final by the Milwaukee Bucks and had a 16-game winning run ended by the Spurs as they exited this season’s competition.
Oklahoma City had equalled the best 25-game start to a season when they went 24-1 by beating the Phoenix Suns in the quarter-finals, but they have now made the second best start to a campaign after 26 games as the Golden State Warriors were 25-1 at the same stage in 2015-16.
Shai Gilgeous-Alexander, the NBA’s reigning Most Valuable Player, top scored for the Thunder against the Spurs with a game-high 29 points.
San Antonio will play the New York Knicks in the final after they beat the Orlando Magic 132-120.
Jalen Brunson scored a season-high 40 points to inspire the Knicks to victory, while team-mate Karl-Anthony Towns added 29 points.
“We came out here and did exactly what we said we wanted to do, which was play Knicks basketball,” said Towns.