Oldham's relegation worries deepened as they slumped to a dismal 3-0 home defeat against Southend.The Latics were let down by woeful finishing before Simon Cox's second-half double inspired Southend to their fourth away win of the season.The Shrimpers needed just five minutes to break through as Shayon Harrison headed a corner against the post and Theo Robinson tapped in the loose ball.Oldham's Eoin Doyle could have levelled before Southend threatened again when Stephen McLaughlin forced a save from Johny Placide.The home side controlled the rest of the first half, but visiting keeper Mark Oxley did well to grab Doyle's close-range header and several more chances went begging.Southend sprung out of defence to double their lead in the 52nd minute, Cox firing in at the second attempt after his initial shot was only partially-saved by Placide.Another clinical attack made it 3-0 just four minutes later as Sam Mantom's left-wing cross gave Cox the simplest of finishes.Jason Demetriou hit the post for Southend, while Oldham's Gevaro Nepomuceno did the same in stoppage time.