The German's victory, his second in successive years on the circuit, came at the expense of Switzerland's Alain Menu, who was in pole position but slipped back to fourth place on the opening lap in his Renault Laguna.
Winkelhock, who was second on the grid, led the pack into the first corner. He was chased by the fast-starting Alfa Romeo of Giampiero Simoni and Paul Radisich's Ford Mondeo.
Menu overhauled Radisich and Simoni to take second place on the fifth lap but could not get on terms with Winkelhock and finished three seconds adrift.
Radisich's title hopes suffered a knock when his engine blew up six laps from home. Third place went to Gabriele Tarquini, the championship leader, who thus extended his lead in the series over the New Zealander.
Simoni finished fourth just ahead of Roberto Ravaglia in the second BMW, while John Cleland, the top home driver on the grid, disappointed.
Though he escaped injury, Cleland spun his Vauxhall on the opening lap and struck the barriers. He was joined in retirement by Andy Rouse, Ford's other contender, who was forced out of the race with mechnical trouble.
With six races remaining in the championship, Tarquini enjoys a 30-point lead over Radisich, with Menu closing up in third place.
AUTO TRADER RAC BRITISH TOURING CAR CHAMPIONSHIP Round 15 (Oulton Park, 16 laps, 44.4 miles): 1 J Winkelhock (Ger, BMW 318i) 28m 09.63s (94.60mph); 2 A Menu (Switz, Renault Laguna) +2.95s; 3 G Tarquini (It, Alfa Romeo 155) +8.61s; 4 G Simoni (It, Alfa Romeo 155) +9.03s; 5 R Ravaglia (It, BMW 318i) +9.31s; 6 T Harvey (GB, Renault Laguna) +16.66s; 7 J Lammers (Neth, Volvo 850) +20.10s; 8 P Watts (Peugeot 405) +29.95s; 9 J Bailey (GB, Toyota Carina) +34.73s; 10 K Odor (Nissan Primera) +34.92s.
Championship positions: 1 Tarquini 210 pts; 2 P Radisich (NZ, Ford Mondeo) 180; 3 Menu 148; 4 J Cleland (GB, Vauxhall Cavalier) 146; 5 Watts 88; 6 Simoni 78. Manufacturers: 1 Alfa Romeo 216; 2 Ford 205; 3 Vauxhall 183; 4 Renault 161; 5 BMW 160; 6 Peugeot 140.
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