That's a great question and I look forward to the responses. The first three that come to mind are Jonathan Cooper (North Carolina) who will require an early pick (round 1-2), Dallas Thomas (Tennessee) who is an OT that will probably move to OG at the NFL-level and may go in rounds 2-3, and Omoregie Uzzi (Georgia Tech) who we may be able to wait till rounds 4-5 to get.
I've seen references of moving John Jerry back to RT (our good friend Omar Kelly) which makes OG a huge need this offseason, and Incognito is in the last year of his deal. The Bills are in a bad cap situation, so Levitre may be a real possibility. We need guards that can get o the second level.