The sixth-placed Shrimpers had influential midfielder Ryan Leonard sent off 20 minutes from time when the game was still goalless.
But strikes from Nile Ranger and Theo Robinson and a stoppage-time penalty by Anthony Wordsworth saw Phil Brown's side secure a fourth successive victory.
Oldham slightly shaded an uneventful first half but failed to seriously test the Shrimpers debutant goalkeeper Christian Walton.
Latics would have fancied their chances when Leonard was dismissed for a late challenge on Ousmane Fane but the dismissal seemed to fire up Southend.
Ranger broke the deadlock in the 75th minute when he netted for the fourth game in succession, tapping home an accurate cross from Robinson.
Three minutes later, Robinson headed home a right-wing free-kick from Wordsworth before Wordsworth completed the scoring from the penalty spot in stoppage time.
The penalty was awarded after Peter Clarke brought down Jermaine McGlashan and Wordsworth made no mistake with a firm effort from 12 yards.
Match report supplied by the Press Association.
Match ends, Southend United 3, Oldham Athletic 0.
Second Half ends, Southend United 3, Oldham Athletic 0.
Goal!  Southend United 3, Oldham Athletic 0. Anthony Wordsworth (Southend United) converts the penalty with a left footed shot to the bottom right corner.
Penalty Southend United. Jermaine McGlashan draws a foul in the penalty area.
Penalty conceded by Peter Clarke (Oldham Athletic) after a foul in the penalty area.
Connor Ripley (Oldham Athletic) is shown the yellow card.
Attempt missed. Oliver Banks (Oldham Athletic) right footed shot from outside the box is too high.
Theo Robinson (Southend United) wins a free kick in the defensive half.
Foul by Ryan McLaughlin (Oldham Athletic).
Jermaine McGlashan (Southend United) wins a free kick on the right wing.
Foul by Anthony Gerrard (Oldham Athletic).
Nile Ranger (Southend United) wins a free kick in the attacking half.
Foul by Peter Clarke (Oldham Athletic).
Attempt saved. Michael Ngoo (Oldham Athletic) left footed shot from outside the box is saved in the bottom right corner.
Foul by Nile Ranger (Southend United).
Tope Obadeyi (Oldham Athletic) wins a free kick on the left wing.
Attempt missed. Lee Erwin (Oldham Athletic) right footed shot from outside the box is high and wide to the left.
Substitution, Oldham Athletic. Aiden O'Neill replaces Paul Green.
Nile Ranger (Southend United) wins a free kick on the left wing.
Foul by Oliver Banks (Oldham Athletic).
Attempt missed. Lee Erwin (Oldham Athletic) left footed shot from outside the box is just a bit too high.
Nile Ranger (Southend United) wins a free kick in the defensive half.
Foul by Peter Clarke (Oldham Athletic).
Substitution, Oldham Athletic. Ryan Flynn replaces Josh Law.
Substitution, Oldham Athletic. Michael Ngoo replaces Ousmane Fane.
Goal!  Southend United 2, Oldham Athletic 0. Theo Robinson (Southend United) header from the centre of the box to the bottom left corner. Assisted by Anthony Wordsworth with a cross.
Ryan McLaughlin (Oldham Athletic) is shown the yellow card for a bad foul.
Jermaine McGlashan (Southend United) wins a free kick on the right wing.
Foul by Josh Law (Oldham Athletic).
Goal!  Southend United 1, Oldham Athletic 0. Nile Ranger (Southend United) right footed shot from the centre of the box to the bottom right corner. Assisted by Theo Robinson.
Substitution, Southend United. Michael Timlin replaces Simon Cox.
Attempt missed. Oliver Banks (Oldham Athletic) left footed shot from outside the box misses to the left.
Ryan Leonard (Southend United) is shown the red card.
Foul by Ryan Leonard (Southend United).
Ousmane Fane (Oldham Athletic) wins a free kick in the defensive half.
Ben Coker (Southend United) is shown the yellow card.
Paul Green (Oldham Athletic) is shown the yellow card.
Ben Coker (Southend United) wins a free kick in the attacking half.
Foul by Peter Clarke (Oldham Athletic).
Attempt missed. Simon Cox (Southend United) right footed shot from the centre of the box is just a bit too high.
Ten-man Southend dug deep to secure a dramatic win over Oldham at Roots Hall.