The Tykes, still to win at home in the Championship this term, hit a post after just 55 seconds through David Perkins and that set the tone for the afternoon.
The ever-busy Jacob Butterfield, captaining Barnsley for the first time, wasted a gilt-edged opportunity before Marvin Sordell, completely against the run of play, was presented with an easy tap-in to hand the Hornets a half-time advantage.
Keith Hill's side continued to boss matters after the break and finally drew level in the 57th minute when Andy Gray headed home from close range.
But neither team, with just one win apiece going into the game, could force a winner, and Watford must take plaudits for a resilient display.
Barnsley were forced into two changes after striker Danny Hynes sustained a hamstring injury in the pre-match warm-up, on-loan Middlesbrough winger Cameron Park coming in for his Tykes debut, with Jay McEveley the other new face.
Hornets boss Sean Dyche named an unchanged XI for the third successive match having registered Watford's first win of the season and his first since taking charge over in the summer with last weekend's 2-0 triumph at Reading.
But the celebrations of seven days ago were a distant memory within the first minute at Oakwell as Perkins fired a shot from the edge of the penalty area which rebounded off a post, hit Hornets goalkeeper Scott Loach and went behind for a corner.
Both sides fired wayward strikes off target amid a bright start to proceedings.
But it was the hosts who were posing the far greater threat in attack and Danny Drinkwater went close when his 25-yard strike drifted narrowly wide of Loach's post.
Further efforts flew over the crossbar at either end, before Butterfield wasted a clear-cut chance to hand the Tykes a deserved lead.
Jim O'Brien's ball into the box was deflected into the midfielder's path but, with the goal at his mercy, the Barnsley skipper could only tap over from close range.
Watford then took a surprise half-time lead. Adrian Mariappa's long ball forward was headed on by Mark Yeates and Sordell was presented with an easy tap-in amid some sloppy defending.
The pattern of the game continued after the restart as the hosts continued to dominate, Butterfield with two early efforts from long distance.
And Hill's side finally got the reward they deserved after 57 minutes as Gray evaded his markers to head home O'Brien's early cross.
Bobby Hassell's surging run was followed by a disappointing finish before the central figure of Butterfield saw his strike blocked by Martin Taylor.
The hosts' endeavours stalled over the final 20 minutes and it was Watford who were left the happier of the two sides come the final whistle.