The team from close to the Welsh Borders chose to bat first on a good track and made a fast start through skipper Warrick Fynn and Joe Carrasco.
They added 40 before Matt Davison rearranged Fynn’s stumps when on 25.
Carrasco was joined by Josh Darley and the score had moved to 112 when Will Mashinge had the opener caught one short of his half-century.
Roman Walker immediately increased the run rate as all of Leamington’s bowlers suffered.
Darley made a patient 34 and Josh Coleridge a rapid 29 before Mashinge dismissed Walker for a fine 95 which included 17 boundaries.
Oswestry closed on an imposing 286 for five, with Mashinge and Davison having picked up two wickets apiece.
Despite an early wicket, Leamington made a promising start, reaching 72 before Waqar Ahmed was caught and bowled by Carrasco for 28.
Jon Wigley was Carrasco’s next victim for a well-made 51, bringing Mashinge to the crease.
He received vital support from Rob Williams (19) and Nabeel Asghar whose 40 came at better than a run-a-ball as Leamington progressed to 222 for five.
Mashinge was pacing his innings well and aided by a quickfire 22 from Chris Calcott, Oswestry’s total remained within reach.
The first ball of the 50th over disappeared over mid-wicket for six, leaving Mashinge unbeaten on 97 and Leamington home by three wickets.
It completed a fine season for Zimbabwean Mashinge who finished the league’s third highest run-scorer having compiled 768 runs at an average of 54.86, while also claiming 27 wickets.