The front man, on loan from Leeds, saw his right wing cross curl past visiting keeper Les Pogliacomi and creep in at the far post. The goal denied Oldham, probably the better side, three much needed points.
Yet it was Barnsley who started the better side and they were ahead after just five minutes. A long free kick from Paul Reid was headed on by Antony Kay and Johnson headed home, wrong footing the Latics keeper.
The home side should have doubled their lead on 15 minutes when a Robbie Williams cross was knocked back in to the danger area for Aussie midfielder Jacob Burns, but he fired straight at the keeper from six yards.
Another chance went begging for Barnsley just after the half-hour mark when Michael Chopra raced clear and with just the keeper to beat he saw his weak effort well saved.
Oldham, who had threatened on a number of occasions then made Barnsley pay on 37 minutes when home goalkeeper Scott Flinders misjudged a cross from Marc Tierney and Chris Killen sent a looping header goalwards which dipped just under the bar.
Boosted by their equaliser Oldham took control in the second half and, after Killen and David Eyres have both seen shots parried by Flinders, former Barnsley reserve team player Luke Beckett gave the Latics the lead when he slotted home from close range in the 66th minute.
Barnsley changed to 3-4-3 for the last 15 minutes and immediately drew level when Johnson's cross beat the keeper for a lucky equaliser.