If you then plaster with staggered joints between OSB and plaster, I think it's fine.
Use 32mm plasterboard screws for wood studs according to the plaster's installation instructions in the OSB as if it were a stud you're screwing into.
By the way, doesn't the OSB's length of 240cm match up evenly with the studs at cc60 ...?
edit:
I see now that you wrote gables, I read carelessly and thought the wall against the roof trusses' support legs. :blushing:
But why don't you install the OSB standing then, with staggered joints against the plaster?