Patrick Schmidt's late, late strike was enough to earn a deserved victory for the home side and leave Forest still needing a point to confirm their spot in the Sky Bet Championship play-offs.
Sabri Lamouchi made three changes from the midweek draw with Swansea as injury to Brice Samba saw him replaced by Jordan Smith while Ryan Yates and Sammy Ameobi were replaced in the starting line-up by Tiago Silva and Michael Dawson.
The home side were in desperate need of three points to retain their Championship status and Smith had to be alert in the early minutes to produce two smart low saves to deny Alex Mowatt and Luke Thomas before Lewis Grabban dragged a shot wide at the other end.
Forest had a couple of bites at the cherry to take a 15th-minute lead as Silva's free-kick from the left caused chaos inside the Barnsley penalty area but the ball wouldn't sit kindly for Joe Worrall and Tobias Figueiredo to strike at goal and the hosts were able to clear the danger.
It was proving to be a testing first half-hour at Oakwell for The Reds and they almost went behind to a scorching strike from Luke Thomas. The Barnsley midfielder hit a crisp drive towards goal from the edge of the penalty area but it clipped Smith's crossbar and went behind for a goal-kick.
Forest were firmly on the back foot and Smith was there once again to bail The Reds out a minute before half-time. Jacob Brown sprung the offside trap and raced into the penalty area but Smith came out to meet him and was able to smother the ball before Brown could pick his spot.
The opening stages of the second half were more even with both sides not looking to show their hand too early. Lamouchi made a double change by bringing on Yates and Ameobi for Silva and Nuno Da Costa as he sought to wrestle control of the contest.
Barnsley came close to opening the scoring midway through the second half when Marcel Ritzmaier took aim from the edge of the penalty area but his deflected strike was touched onto the post by Smith and the ball span behind for a corner.
The home side knew they had to throw the kitchen sink at Forest to get the victory they needed. Mowatt's free-kick into the penalty area was headed up but not away by Forest and as substitute Matty Wolfe headed the ball towards goal from close range, Worrall was there to clear it off the line.
Apart from score, Barnsley were doing everything else and they almost broke the deadlock with ten minutes to play. A short corner was delivered to Jordan Williams on the edge of the penalty area and he drilled a powerful effort inches over Smith's goal with Forest keeper at full-stretch.
The goal had been coming all afternoon and it duly arrived in the fourth minute of stoppage time. An almighty goalmouth scramble inside the Forest penalty area ended with the ball being turned home at close range by Patrick Schmidt to ensure The Reds will still need a point on Wednesday night against Stoke City to secure their play-off place.
Forest: Smith; Jenkinson (Benalouane 85'), Figueiredo (Mighten 90'), Worrall, Dawson (c), Ribeiro; Watson, Sow, Silva (Yates 55'); Da Costa (Ameobi 55'), Grabban
Subs not used: Shelvey, Bostock, Lolley, Darikwa, Walker
Booking: Watson 51'
Barnsley: Walton; Ludewig (Woodrow 46'), Sollbauer, Andersen; J.Williams (Halme 87'), Mowatt (c), Thomas (Wolfe 67'), Ritzmaier (Schmidt 82'), Styles; Brown, Chaplin
Subs not used: Collins, B.Williams, Bahre, Oduor, Marsh
Goal: Schmidt 90'
Referee: Scott Duncan
