I have a question. Did they film all the island stuff ahead of time, before they started filming the present day stuff? I'd have to guess yes, since it looks like it was all filmed at one time and Oliver seems to have his real hair longer, and not wearing a wig or something. Its just something i've been thinking about recently.
I'm pretty sure they film the island scenes second unit as the series proceeds. They may double up filming island segments for convenience but there's no way they could film all of the island scenes for 22 episodes in one giant lump.
As for Oliver's "hair", his wig for the flashbacks is one of the worst I've ever seen in a professional TV production. The Vancouver humidity is doing it no favors, as well. Every time Ollie pulls off one of those ski masks, I expect the wig to come with it.