The first half was fairly even with both teams playing good football and Scunthorpe also went agonisingly close to equalising Alessandra's first goal.
But Krystian Pearce hit the woodwork and within a minute Oldham had doubled their lead prior to making a hatful of other chances.
The home side had a goal controversially ruled out in the first ten minutes when Lee Hughes ran clear and beat keeper Joe Murphy, only to be pulled back by an offside flag.
Hughes and Danny Whitaker went close before lively Scunthorpe striker Gary Hooper dragged his angled shot just wide.
Oldham broke through on 26 minutes when Alessandra took a pass from Whitaker 25 yards out and saw his shot deflected, leaving Murphy helpless to prevent the teenager's second goal of the season.
Hughes went close twice but Scunthorpe nearly hit back either side of half-time as Hooper tested Mark Crossley, before Pearce went up for a corner and watched in agony as his header crashed against the bar.
Oldham scored from the next attack as right-back Neil Eardley crossed to the near post and Alessandra finished neatly from six yards.
The hosts took complete control and Alessandra finished off his hat-trick on the hour mark with the best of the lot, as he arrowed an angled drive inside the far post from 16 yards from Eardley's pass.
Hughes, desperate the join the party, missed three good chances and Chris Taylor was denied by the post as the Iron left Boundary Park with a surprisingly severe setback.