Can I replace just part of a 1x6 roof deck with OSB?
You certainly can. I've done just that with a 1950s home I owned to patch in where the chimney came through and to replace rotted boards across an entire section. Just use spacers between the OSB sheets as is common practice to create 1/8" gaps. (Or use 16d nails as spacers with 3/4" sheathing--the sheets are sized for that.) 

Also, use suitable fasteners. 8d nails are good for 1/2" material, but you might go a bit larger (10d or 12d) for 3/4" material.