The championship leader pulled away from Ducati’s Shane Byrne in the closing stages, with team-mate Luke Mossey trailing home in third.
Jason O'Halloran, Peter Hickman, James Ellison, Josh Brookes, Christian Iddon, Glenn Irwin and Jake Dixon completed the top 10.
Haslam moves onto 111 points in the overall standings, with Mossey (97) and Iddon (73) his closest challengers. Five-time champion Byrne is eighth on 33 points.
The race was brewing into a battle between Haslam and Byrne, but it failed to spark as the Ducati man settled for second ahead of race two.
Byrne’s team-mate Glenn Irwin avoided an accident after stabilising a huge wobble on lap 12 as the race passed without major incident.
Read the original article on Eurosport: Haslam holds off Byrne to win race one at Oulton Park