I think it would be difficult to find a precise number, but guessing that its not very high.
There are always job openings because docs are constantly leaving bad situations for slightly better ones.
ie OD with a Sam's Club lease averages 2 patients/day jumps to a Sears Optical, hoping it will be better.
Underemployment however is a serious issue.
Many OD's are working less than 4 days/week & many are getting by doing fill-in work at multiple locations.
Working 3 days/week with no benefits and a $150+K student loan is a bad combination!