Double garage flat roof repairs contd...

Thanks to all who have offered advice already.
I'm getting conflicting views about deck material; some say 18mm OSB3 is fine others say where the underside is exposed to garage type atmosphere that OSB3 is definitely not to be recommended. Instead one should use exterior WBP (presumably still 18mm). I am assured also that this can be sourced at a similar price, i.e. Wickes OSB3 is £15 a standard 2440x1220 sheet for 10+. Searching the internet doesn't come up with prices anywhere near that. More like twice the price.
Is there a correct answer to this or is it a matter of opinion?
Thanks. PeterM

hi mate do not use osb its crap any money you try save now will cost you later on if you are felting the roof you will get away with using 18mm cdx (basic ply) doesnt have to be marine or external as long as you will be finishing it quickly and not leaving it exposed to the elements. i use it all the time silicone between all joints and screw every 9" it will be fine

Nonsense, OSB is absolutely fine and will withstand a soaking, it will only be crap if you do a crap roofing job on top of it.

we use osb to and if your roof dosnt leak osb is fine wbp ply will do the same job tho purely down to choice