I'm going to reinforce a 2.2m span of shelf made out of 18mm OSB bord with two 60*4 mm steel strips running underneath the board.
My understanding is that connection points would mostly have sliding forces when the shelf is bent under load, but I'm not sure.
I'll answer this from a structural engineer's standpoint (being that I am one).
What you're doing here is making a composite shape out of your wood shelf with steel reinforcement. As this shape bends, in order to behave compositely, you need to restrain the relative slipping of the OSB layer over the steel layer and develop what's called shear flow between the two layers.
The math behind the answer is beyond the scope of this SE, but I'll try to give you a belt-and-suspenders answer.
Screws are obvious answer, what size should I get and how many? Or would glue or rivets suit me better?
You have 18mm (~ 11/16") thick OSB and 4mm (~ 3/16") thick steel straps for a total depth of 22mm (~ 7/8"). I would go with 19mm (3/4") long round head or pan head screws, say a #8 or #10 screw size, spaced at 100-150mm (4"-6") on center. You'll have to drill through the steel strap, obviously.
Personally, I'm a fan of SPAX screws for OSB since they have an aggressive, self-drilling thread pattern. They make a pan-head screw that would be just about perfect.
Most glues will not form a long-term or reliable bond between steel and wood, so I would not recommend that. Rivets might work, but I'm not too optimistic about their long-term holding power in wood compared to screws (and they're harder to install).
@Ashlar Yes, to stiffen the shelf. Max load is 100kg more or less evenly distributed. Shelf's depth is 1m.
I would space the straps at the approximate third points of the depth of the shelf (i.e., 33cm from either the front or back).
Edit: I just did a quick back of the envelope calculation on your shelf. The shear flow between the two layers will be on the order of 200 lb/in (or 100 lb/in per strap). So, the #10 by 3/4" screws at 4" centers should work. See this PDF for screw shear capacities in plywood.