How to cover a rectangle with least number of convex polygons cut from sheets of given size
I suggest dividing the 3 m length into three even 1 m lengths. Then divide the 2.6 m width into two uneven lengths. If you want to minimize cuts I suggest 1.25 m and 1.35 m. These add up to 2.6 m. 

Out of a single 1.25 x 2.5 board you can cut out one 1.00 x 1.25 piece and one 1.00 x 1.35 piece. It will require 3 straight cuts (labeled in order below). 



Repeat three times and assemble. I've drawn two versions of the assembly. Which one you prefer may depend on the framing that the OSB is attached to and what you intend to cover it with, if anything.