Ouseburn hold an 18-point advantage over Birstwith and require one more win in their four remaining matches to raise the trophy.
Sam Parker smashed an impressive century and shared a 160-run partnership with captain David Cummings (95) as Ouseburn scored an unreachable 289-4 on Saturday.
Parker hit 13 boundaries and a six in his 111 not out, which saw him also put on 61 with fellow opener Adam Fisher (24) and 66 with Jamie Bryant (45).
In a rain-reduced fixture, Hampsthwaite gave a strong account of their batting skills and totalled 202-3 from 38 overs.
Captain Sam Dabin remained unbeaten on 84, while opener James Cousen hit 48.
Birstwith’s title chances were washed down the drain in a 78-run loss at Helperby.
Reduced to 41 overs per side, Helperby amassed 200-7 with Trevor Chibvongodze (72) and Luis Rees-Hughes (60) compiling the early impetus.
Batting has been Birstwith’s strength in the 2016 summer but they struggled against the Helperby attack of Dan Marston (3-37), Joe Corner (2-33) and Matt Cavanagh (2-22).
The challenge of long-term pursuers Darley came to a crashing halt at neighbours Pateley Bridge after they were bowled out for 139, the hosts sealing a four-wicket win.