The winger, who re-joined the club over the summer, rifled a 20-yard drive into the top left corner of the goal after 54 minutes to cancel out Cameron Burgess' early opener.
Latics were reduced to 10 men four minutes after Hall's strike when Ousmane Fane fouled Marvin Johnson and was sent off for his second yellow card.
And later, Oxford boss Michael Appleton and a member of Oldham's coaching staff were both sent from the dug-out for remonstrating over a booking.
On-loan Fulham centre-half Burgess fired Oldham in front in the 10th minute.
Keeper Simon Eastwood failed to collect Ollie Banks' free-kick, and the ball ran loose for Burgess to knock into an empty net from five yards.
Almost immediately, Kane Hemmings should have levelled as he slid in to meet Chris Maguire's cross, but diverted the ball over the net from four yards.
Oxford dominated the second half and Connor Ripley saved well from Chris Maguire, Marvin Johnson twice and Chey Dunkley as Latics held out for a point under severe pressure.
Report supplied by the Press Association.
Match ends, Oxford United 1, Oldham Athletic 1.
Second Half ends, Oxford United 1, Oldham Athletic 1.
Corner,  Oxford United. Conceded by Cameron Burgess.
Substitution, Oxford United. Ryan Taylor replaces Kane Hemmings.
Connor Ripley (Oldham Athletic) is shown the yellow card.
Corner,  Oxford United. Conceded by Charles Dunne.
Attempt missed. Marvin Johnson (Oxford United) right footed shot from the centre of the box is too high.
Attempt saved. Marvin Johnson (Oxford United) left footed shot from the left side of the box is saved in the top left corner.
Substitution, Oldham Athletic. Brian Wilson replaces Ryan McLaughlin.
Attempt missed. Chris Maguire (Oxford United) left footed shot from outside the box is just a bit too high.
Substitution, Oxford United. Tyler Roberts replaces Robert Hall.
Philip Edwards (Oxford United) wins a free kick in the defensive half.
Foul by Ryan McLaughlin (Oldham Athletic).
Foul by Cheyenne Dunkley (Oxford United).
Lee Erwin (Oldham Athletic) wins a free kick in the defensive half.
Philip Edwards (Oxford United) wins a free kick in the attacking half.
Foul by Lee Erwin (Oldham Athletic).
Attempt missed. Robert Hall (Oxford United) left footed shot from the centre of the box is close, but misses to the right.
Ryan McLaughlin (Oldham Athletic) is shown the yellow card for dangerous play.
Philip Edwards (Oxford United) wins a free kick in the defensive half.
Foul by Ryan McLaughlin (Oldham Athletic).
Delay over. They are ready to continue.
Delay in match  (Oxford United).
Attempt missed. John Lundstram (Oxford United) right footed shot from the left side of the box is just a bit too high.
Corner,  Oxford United. Conceded by Connor Ripley.
Attempt saved. Cheyenne Dunkley (Oxford United) header from the centre of the box is saved in the top right corner.
Corner,  Oxford United. Conceded by Connor Ripley.
Attempt saved. Marvin Johnson (Oxford United) left footed shot from outside the box is saved in the bottom right corner.
Attempt missed. Marvin Johnson (Oxford United) right footed shot from the left side of the box is just a bit too high.
Substitution, Oxford United. Josh Ruffels replaces Alexander MacDonald.
Corner,  Oxford United. Conceded by Connor Ripley.
Attempt saved. Chris Maguire (Oxford United) right footed shot from the right side of the six yard box is saved in the bottom right corner.
Substitution, Oldham Athletic. Lee Croft replaces Ryan Flynn.
Attempt saved. Ryan Ledson (Oxford United) left footed shot from outside the box is saved in the top centre of the goal.
Foul by John Lundstram (Oxford United).
Paul Green (Oldham Athletic) wins a free kick in the defensive half.
Delay over. They are ready to continue.
Robert Hall (Oxford United) is shown the yellow card for a bad foul.
Oliver Banks (Oldham Athletic) is shown the yellow card for a bad foul.
Robert Hall (Oxford United) wins a free kick in the attacking half.
Rob Hall scored his first goal since returning to Oxford with a brilliant equaliser in an ill-tempered draw with struggling Oldham at the Kassam Stadium.