
Osei's exquisite finish after just 34 seconds broke the deadlock for Ashton United, however, just three minutes later Town were level from the penalty spot following a challenge on Joseph Gibson inside the penalty area.
Whitby, though, were unable to build on this leveller thanks to Osei's deadly header in the second half.
Nathan Haslam made three changes to the side that had lost by the same scoreline to Guiseley in midweek as Aaron Haswell, Jassem Sukar and Frankie Whelan came in for Sam Collins, Alfie Doherty and Priestley Griffiths.
The Seasiders made the worst possible start to proceedings when Osei capitalised on a mistake from Campbell Darcy in the first minute. Trevor Bryan's cross-field pass slipped under the foot of the on-loan Hartlepool United man and Osei curled an effort into the far top corner of Shane Bland's goal.
But, just three minutes after conceding, Town were given the chance to level matters when Gibson was felled inside the area. Stephen Walker, who had netted every penalty kick he had taken so far in the 2024/25 season, stepped up to send Jordan Eastham the wrong way from twelve yards.
It was Ashton who went on the front foot from there on in however; Bryan hitting the target, curling an effort into the hands of Bland before Osei struck wide with an acrobatic effort.
Bryan then headed over following an Ashton attack before James Hardy was denied by a strong block from Whelan.
Whitby did show some signs of life though when Jake Charles saw his left-footed strike blocked at the other end but, on the whole, Town were restricted to opportunities on the counter.
Lewis Hawkins could only find the hands of Eastham with a tame effort before Bland had to make a superb one-handed stop to tip Jack McCourt's long-range strike over his crossbar on the stroke of half time.
After the break, McCourt came close again as he curled a strike off target from range before Jason Gilchrist though he had put his side back in front, only to be denied by the offside flag. Bland, in particular, was a relieved man having initially made a brilliant save to deny Osei.
Bernardo Couto also called Bland into action with a driven effort, but there was nothing the Whitby gloveman could do in the 58th minute as Osei headed in his second goal of the afternoon when he got on to the end of a cross from the right wing.
In the 72nd minute, Hardy looked to extend the advantage for Steve Cunningham's men, however he stroked a low strike wide before Ashton looked to shut up shop.
Their tactic worked as Whitby were restricted to one real chance of note when Walker advanced forwards and saw a chance to shoot, only to drag his effort wide.
But, in the end, it was ultimately another disappointing afternoon for the Blues who slumped to a fifth straight league defeat.