James Warrington also saved a second half Lee Allinson penalty as Slough doggedly held on to claim all three points after the Waders had controlled much of the opening period.
Visitors Biggleswade, who beat bottom side Woodford 10-0 in their previous league match, harbour real hopes of promotion, and looked full of attacking intent. They took the lead in the 12th minute when Joel Mason instinctively snapped up a chance after a corner was swung in.
The away side went close shortly after through Brett Donnelly, who weaved his way through but shot wide.
Slough penalty appeals were waved away as Parsons went down after a heavy challenge in the box, but play continued and Warrington was called into action to parry away a volley from the edge of the area from Mason, which no Biggleswade player could follow in.
Slough's first sight of goal did not arrive until around halfway into the first half, O'Toole seeing a shot deflected over. The Waders looked on top, with Slough playing into their hands somewhat with attempted balls over the top being dealt with comfortably.
The home side came into the game though, and after Kyle Anthony headed Parsons' corner narrowly over the bar, Slough went agonisingly close as neither Brown nor Dan Burnell could get a touch on Anthony's cross-shot to turn it home.
Two minutes before half time, Slough's spell of pressure paid off when Danny Murphy swung a corner in, which goalkeeper Michael McEntegart missed, and Brown got the telling touch to turn the ball in.
Within a minute, Slough were ahead as Jake Parsons ran onto a flick-on to fire past McEntegart and leave Biggleswade stunned going into the interval - a real turning point in the game.
Warrington made a crucial save with his legs early in the second half to keep his side's advantage, but Slough picked up where they left off in the first half and began to exert more pressure on the visitors and, in the 55th minute, hassled them into a mistake which led to the third goal.
Slough won the ball 40 yards out and Dave Deeney fed Burnell, who played in O'Toole on the left side of the box, and he coolly poked the ball past McEntegart with the outside of his right boot to extend the lead. The striker has now scored in each of his last five games, notching seven goals in the process.
As the game opened up, Warrington again made a good diving save to keep out a free kick, and Slough broke away in search of a fourth, but Burnell was denied by a great point-blank save from McEntegart after Anthony had cut the ball back.
Biggleswade had the chance to reduce the arrears when they were awarded a penalty, but Warrington guessed correctly and made the save from Lee Allinson's effort, with Slough eventually able to clear their lines. The Rebels looked fortunate to escape a red card for the already-booked Alex Brown, who seemed to have committed the foul, with Ashley Deeney cautioned instead.
Slough soaked up late pressure from Biggleswade well, Warrington saving well from capitain Steven Gentle and seeing other hopeful efforts from distance go off target, and the home side claimed what could be a crucial three points in the promotion chase.
Biggleswade Town Line up
Michael McEntegart, Max York, Andrew Iwediuno, Peter Gatti, Steven Gentle (c), Lee Allinson, Craig Daniel (Paul Barnes), Joe Reynolds (Alex O'Brien), Brett Donnelly, Joel Mason, Jon Woolf. Subs not used: Luke Knight, Ricky Case, Kevin Kilroy.