George Moncur put the U's ahead but a double from Ashley Fletcher put promotion chasing Barnsley 2-1 up.
Taking charge for the first time, David Wright made two changes, Moncur and Lee replacing Porter and Dunne.
After another superb performance between the sticks, Elliott Parish was stretchered off and replaced by Macauley Bonne in the second half, with Tom Lapslie scoring a late leveller in the seventh minute of injury time.
The U’s were straight out of the blocks at Oakwell, with the home woodwork being hit as early as the second minute.
Owen Garvan found Elliot Lee out on the left and his pull back led to George Moncur sweeping a shot that hit the inside of the post before it was cleared.
Moncur went close again on nine minutes but his lobbed effort from a narrow angle went narrowly wide after a cross from Nicky Shorey.
At the other end, Winnall should have done much better from a corner routine, but skied his effort over the stand from twelve yards out.
U’s keeper Elliott Parish made a couple of comfortable saves from a long range effort, before keeping the score goalless with a great double save.
The first was the best, a fantastic instinctive reflex save when Winnall guided a cross goalwards from only a few yards out.
From the resultant corner, Parish dived full length to turn away a header.
On thirty five minutes, it was Barnsley’s turn to hit the woodwork, with Mawson’s header bouncing up and clipping the bar on its’ way over.
Moncur almost opened the scoring on thirty seven minutes, but his first time effort was blocked – it was a superb move from the U’s, with Garvan and Shorey slicing through the Barnsley defence.
Five minutes later, he did open the scoring, with Lee proving the ammunition.
Lee won a tackle to regain possession centrally, before breaking forward and slipping the ball through to Moncur.
Moncur’s first touch completely wrong footed the last defender and his second was a calm side foot finish into the bottom corner, giving Adam Davies no chance.
Less than sixty seconds later, Lee almost doubled the lead, but his shot from the edge of the box was deflected just wide.
That was the final chance of the first half, with the U’s good value for their 1-0 lead.
As in the first half, the U’s had the first effort of the second, with Lee cutting inside before shooting straight at Davies.
The hosts started to come into the game, and just before the hour mark they got back on level terms through Ashley Fletcher.
Shorey had a pass intercepted, Fletcher beat Tom Eastman and then rifled his shot past Parish and into the far corner.
Winnall should have scored a few minutes later, but he miskicked right in front of goal as Barnsley looked for a winner to keep their place in the Play-Off spots.
On sixty five minutes, Alex Wynter replaced the injured Tom Eastman, and Alex Gilbey entered the fray minutes later, with Massey coming off..
Halfway through the second period, Parish was again in the action, with a save from a Roberts’ header.
Moncur combined with Lee on seventy minutes, but saw his curler go just over the crossbar.
Wright made his third and final substitution on seventy six minutes, with Drey Wright coming on for Lee.
With ten minutes to go, Barnsley went into the lead, with Fletcher again the scorer.
A cross from dangerman Hammill found sub Chapman, his effort was saved but looped up nicely for Fletcher to head into the empty net.
Parish was injured making the save and had to be stretchered off, meaning Macauley Bonne going in goal for the remainder of the game.
The fourth official signalled seven minutes of injury time, with the U’s down to ten men and with an outfield player in goal.
In the seventh and last of those minutes, Tom Lapslie dived to head home the most unlikely of late levellers.
Shorey’s initial cross was cleared but Joe Edwards got the ball back into the near post for young midfielder Lapslie to head home.
It was no more than the U’s deserved, and an action packed first game in charge for David Wright.