Isn't OSB supposed to be more stable to elements comparing to wood? Isn't that what's all the adhesive in it is for.
Not be a wise-guy, but the adhesive is there to hold the little pieces of wood together. Dimensional stability is a secondary and very desirable benefit.
Tables give me tensile strength of OSB few times smaller than wood. Can I replace 18mm x 36mm wooden beam safely with 2* 50mm strips of 22mm OSB screwed and/or glued together? That'd give me 50x44 "beam" section -- should be quite enough.
I assume that you are considering stiffeners of 18mmx36mm strips with the 36mm dimension vertical and the 18mm horizontal and, likewise, the 44mmx50mm OSB strips with the 50mm vertical and the 44mm horizontal. (That's the way that you get maximum beam action.) You would get comparable beam action with the OSB beam solution and the two pieces that make up your 44mmx50mm stiffeners can be screwed - it should not affect the stiffness.
If, when you first place stuff on the shelf, it visibly sags, then use more stiffeners, or deeper stiffeners.
OSB comes in various stiffness depending on the glue and wood. Compared to pine it will sag from about the same or up to twice as much. I would assume that the stiffness varies with price.
You also have an issue of attaching the stiffeners to the shelf. Screwing or nailing the edges of the stiffeners to the shelf material would generally be discouraged, but they might hold well enough to hold the stiffeners in position. Gluing OSB has serious problems as well - common woodworking glue has trouble adhering to the adhesives (think plastic resins and wax) that hold the OSB together.