Two Oxford shells have both decks on. We are still installing the skegs on the other two.
We used just four screws for alignment, then used duct tape / packing tape to hold the decks down. The only down-side to this, that I can see, is that if you have a lot of overhanging material, the tape tends to make the deck 'hump up' or take a smaller radius than it should. We solved this by pre-trimming most of the excess plywood, thus reducing the possible leverage on the edges.
We used just four screws for alignment, then used duct tape / packing tape to hold the decks down. The only down-side to this, that I can see, is that if you have a lot of overhanging material, the tape tends to make the deck 'hump up' or take a smaller radius than it should. We solved this by pre-trimming most of the excess plywood, thus reducing the possible leverage on the edges.