Considering how much intel he has beyond his fathers notebook to break into specific bank accounts and not to mention being part of the Russian Maffia... Oliver didn't spend 5 years on that island.
He owned it.
Oliver must have been on and off the island building up a sophisticated mechanism for revenger for years, and then engineered his discovery and rescue as it happened, only to have cleverly pretended to have been there the whole time as part of some cunning ruse.
Becoming absolute master of that savage land would have taken a while but after he'd filled up enough massgraves, with the resources they left behind, he only stayed on that island for as long as he wanted to.
Actually just clearing up the evidence of a standing army associated with Checkmate on site, would have at the least required earth-movers if not cargo-ships... But then I'm just imagining that they had tanks on the island for some reason.
But the only reason that they would need tanks, is if there actually were dinosaurs to put down.