Sam Winnall struck another couple of superb goals as the Reds were rewarded handsomely for a dominant performance.
Barnsley probably enjoyed the better of a pulsating and at-times nervy first period.
They did lead at the interval, but only broke the deadlock seconds before the half-time whistle.
At that point Winnall broke down the right before cutting the ball back into Adam Hammill's path. The popular winger converted a shot from 10 yards - via a deflection off Jason Demetriou - to send Oakwell wild.
Earlier Lloyd Isgrove saw a piledriver well saved by Saddlers' 'keeper Neil Etheridge, while the visitors responded with Tom Bradshaw being denied by a smart Adam Davies stop.
The Reds were denied what looked like a clear goal in the 23rd minute.
Isgrove converted a brilliant Hammill cross, only to see the effort ruled out for offside. However, replays indicated that Isgrove had been well onside.
Isgrove went mighty close again three minutes after the restart. He ran half the length of the field before seeing a powerful strike well saved by Neil Etheridge.
However, two goals in as many minutes from Winnall sent Oakwell wild again, with both being splendid individual strikes.
The first, in the 54th minute, was fired home from 20 yards as the Walsall defence retreated.
A minute later, Winnall buried his 24th goal of the season, charging through the middle before slotting past Etheridge from an acute angle.
Etheridge kept out a neat volley from Ashley Fletcher as the Reds pressed for a fourth, but a 3-0 advantage going into Thursday's second leg at the Banks's Stadium ought to be enough.
Barnsley (4-4-2): Davies; Scowen, Roberts, Mawson, G Williams; Isgrove, Brownhill, Hourihane, Hammill; Fletcher, Winnall.
Subs: Townsend, Cowgill, Chapman, Toney, McCourt, Tuton, White.
Walsall (4-1-4-1): Etheridge; Demetriou, O'Connor, Downing, Henry; Chambers; Lalkovic, Morris, Sawyers, Forde; Bradshaw.
Subs: MacGillivray, Taylor, Mantom, Preston, Cook, Osbourne, Hiwula.
Referee: Peter Bankes (Merseyside).