The visitors had the better of the opening exchanges in an open game as Hall, on-loan from Sheffield United, continued to test the home defence with his power and pace only to meet his match in Michael Turner.
Lloyd Owusu should have done better when right-back O'Connor picked out the former Reading man with an excellent cross but he was unable to direct a header under pressure from the defence.
As the game progressed the midfield battle became heated and referee Richard Beeby struggled to keep a lid on the contest as he cautioned players from each side as the tackles flew in.
It took a mistake from the otherwise impressive Turner when he failed to control a routine ball over the top and Hall pounced to run through on goal to slide the ball calmly past the advancing Stuart Nelson to give the visitors a deserved lead.
Owusu always looked Brentford's best way back into the game and he should have equalised minutes before the break when he sprang Chesterfield's offside trap only to be foiled by Barry Roche's feet.
The second half got off to an explosive start when Sam Sodje's diving header was tipped over by Roche just moments before man of the match Mark Allott found Hall in space only to curl the ball just wide of the far post.
Brentford continued to battle and got their just reward when Sodje fed O'Connor inside the box and the full-back made no mistake from close range.