The visitors grabbed their third victory in four outings as they moved up to sixth place in the Betfred Championship.
Oldham opened the scoring with a Ryan Ince try which Dan Abram goaled off the touchline, but Dewsbury levelled with a try in the corner by right-wing Andrew Gabriel and the conversion by Paul Sykes.
It was 6-6 at half-time, but Roughyeds were pinned down at the bottom end of the ground in the second half when Dewsbury dominated down the slope and scored another 18 points to make it a disappointing day for the home club and home supporters.
