Backing up from their 14-goal third term against Brisbane last week, the Giants' Orange Tsunami also overwhelmed the Demons late on Sunday in the 19.5 (119) to 9.16 (70) win. GWS kicked 10 goals to three in the second half.
Melbourne won the inside-50s 62-46 and also were ahead in contested possessions and clearances - key numbers that normally equate to success.
But coach Steven King was left lamenting some terrible clangers that, combined with the Giants' goalkicking accuracy, doomed Melbourne.
In his first game against the Demons, Oliver only had two possessions to quarter-time and Jack Steele was winning their midfield match-up.
But Oliver racked up 28 possessions to be among the Giants' best. Fellow onballer Finn Callaghan was best afield with a whopping 797m gained, 31 possessions and two goals.
A steady stream of Melbourne players went to Oliver at the final siren and the feeling was mutual, after he moved clubs last year.
"No hard feelings, the boys - I love them all, talk to them all the time," Oliver said of the Demons.
Coach Adam Kingsley was rapt that Oliver impressed against his old club.
"It's not often you say he 'only' had six clearances in the end, but that's the kind of form he's been in," Kingsley said.
"They did a good job to start and he worked his way in."
Oliver said plenty of hard work had paid off in the past fortnight, with the Giants now going into their mid-year bye.
"It's pretty good ... just that final piece, we've missed a handball, missed a kick ... the last two weeks, it's come together and hopefully we keep it going," Oliver told FoxSports.
Stephen Coniglio continued their bad injury run, leaving the game in the second quarter with hamstring soreness.
Sam Taylor returned for his first game of the season and sent a scare through GWS when he left the field, but it was a hamstring cramp.
Kingsley said while a spate of hamstring injuries needed investigation, GWS are coping well with their casualty list.
"I feel like we're working through this patch really well, in terms of win-loss, but we're also getting games into young players that's going to help us long-term," he said.
Melbourne are now on a four-game losing streak at Traeger Park and coach King knew exactly where they went wrong against GWS.
"We were really clunky and inefficient in our forward half and off the back of that, (it) probably put us in some compromising positions defensively," he said.
"I haven't seen us use the ball that poorly this year, with some blatant clangers.
"It really hurt us ... we've punished ourselves with the ball."
Toby Greene, Max Gruzewski and Callum Brown kicked three goals apiece for the Giants.
Melbourne star Kysaiah Pickett, who finished with seven clearances, surely has the leading contender for goal of the year with an outrageous snap in the first term.
Late in the opening quarter, Pickett pounced on the ball after it spilled from a marking contest.
He tracked it to the pocket, taking a quick look over his shoulder. Then he snapped with his right foot over the left shoulder from the boundary line for a freak goal that brought Melbourne to within two points at quarter-time.
The Demons have lost two in a row for the first time under King, and Sunday was a hard reality check ahead of their annual King's Birthday blockbuster against Collingwood.
That clash takes on even greater significance for the Demons after the death of former coach Neale Daniher, whose FightMND campaign has raised the King's Birthday game to a major event.
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