Hounding the Southport defence like a hyena throughout, Owusu brought his quarry down with a superb piece of predatory opportunism in the 74th minute.
The result once more underlined Slough's capacity to raise their game after the break and McDermott was never in doubt about the outcome. He said: "We've had occasions over the last 15 games where even when we've been a goal down I and the players have felt we could win the game.
"On Saturday we gave another solid performance - especially in the second half when we always look stronger - and should have run out two or three goals to the good." McDermott, for whom something of a turning point this season came with a first win in six attempts against Northwich Victoria, added the scalp of an experienced, well-balanced, top-six side whom - like the Vics - the Rebels had previously never beaten at Wexham Park
Southport's big guns included ex-Manchester City apprentice Ged Kielty, Paul Futcher, the 41-year-old former Manchester City, Derby and Chester player, and Dave Thompson, a 35-yearold forward with 550 Football League appearances, many with Notts County, Preston North End and Chester.
This trio and Dave Gamble, the longest-serving player at Haig Avenue ensured a first half which - with only one corner resulting and difficult, spongy conditions underfoot - was lively rather than eventful.
The best chance of an even, open half fell, however, to Southport when Thompson, who earlier had finished lamely when faced with an outrushing Paul Wilkerson, planted a header from Paul Jones' cross against the crossbar and Gary McGinnis headed away the follow up from Paul Mitten.
Slough got in close only occasionally with Owusu, Gary Abbott and Danny Bolt probing hard when they did. Abbott tweaked a cross along the bye-line to Owusu which Phil Homer hacked away; Owusu sneaked the ball inside to Abbott whose left-footed chip over Stewart from the edge of the box floated wide; and Owusu twice combined with Bolt who dragged a right foot shot wide before delivering a cross which flashed across the face of goal with Abbott and Brazil just unable to turn home.
But the Rebels turned the screw straight from the restart. A neat back-heel by Abbott then sent Bolt clear and a switch inside to Owusu gave the striker a perfect opportunity, but he struck the shot under pressure against Stewart's legs.
Disappointed but undeterred, Owusu retrieved a ball going out and forced centreback Futcher into conceding a comer from which Leroy Dove cleared Hercules' header off the line. With Wilkerson parrying a long range effort from the power boot of Gamble who also tried his luck direct from a free kick, the Rebels replied with further efforts by Bolt and Hardyman.
With Slough maintaining the pressure Owusu loped after a cross which looked about to be cleared along the edge of the area. A lunge and a poke sent the ball wide to Brazil whose fiery first time shot Stewart was unable to hold onto.
Owusu, as though the rest of his life depended on it, went in for the kill and smashed the ball home from inside the six-yard box. Southport rallied, with sub Brian Ross netting a goal which was disallowed for offside, but Owusu almost sealed the result with a second goal on the stroke of time.
Sent clear this time by West, Owusu was denied by Stewart who, having raced 10 yards outside his area, appeared to handle the ball. A magnanimous McDermott said: "Stewart did handle the ball, but the referee would have had to send him off - so that's fair enough."
Southport Line up
Stewart, Farley, Dove, Kielty, Horner, Futcher, Thompson, Jones, Mitten, Gamble, Formby. SUBS: Moran (77, for Jones). Ross (69, for Mitten), Moore.