Crewe will stay in League One despite producing a shocking defensive display.
Kenny Lunt gave the ball away to Deane Smalley, who smashed in Oldham's first and Neil Eardley beat several defenders before firing past Ben Williams.
Williams kept Oldham at bay until Eardley cut in from the right and fired a 30-yarder into the top right corner.
Gary McDonald poked home a fourth before Steve Jones ran clear to pull back a consolation, and Bournemouth's failure to win meant Crewe were safe.
Crewe: Williams, Abbey, McCready, O'Donnell, Billy Jones (O'Connor 80), Lunt, Schumacher, Roberts, Morgan (Pope 58), Maynard, Steve Jones.
Subs Not Used: fon Williams, Cox, Moore.
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