Oldham, who knocked Liverpool out of the FA Cup last week, fielded three new players but still stuggled against a lively Saddlers who have won five of their last six matches.
Oldham were the better side at the start and after missing a couple of chances Lee Barnard, on loan from Southampton, put them ahead with a fine header from Lee Croft's cross in the 19th minute.
A minute later Walsall should have been level, but Will Grigg failed to beat Dean Bouzanis with a close-range header from Sam Mantom's cross.
The Saddlers did equalise, however, five minutes from half-time when the recalled Andy Taylor found James Baxendale, who cut in from the left and scored his fourth of the season with a low shot from a tight angle.
A minute later Febian Brandy hit the inside of a post with the Latics keeper beaten.
Paterson put Walsall in front for the first time after 74 minutes with a 25-yard drive after good work by Adam Chambers, who also set up Paterson to beat a defender and hit number three for Walsall in stoppage time.