If you have a blood of o+, will it be a lot harder to find a lung donor?

That is not true. O positive group is commonly found all acros the globe.
Probably not. Lung donors should be ABO compatible with the recipient. An o-positive recipient can get blood or an organ from o positive or o negative donors, which would mean about 40% of people could donate. The most common ABO blood group is a, with o the next most common. Rh negative people are about 15% of the us population, with 85% rh positive.