As W/S states , osb is not a good idea for flooring, apart from the fact it is not LOAD bearing rated, (wall or floor sheathing , is the only rating it has!!) also if using it for the roof! then the Bitumen impregnated stuff is best!!
RE: sky lights< if you are thinking in terms of opening skylights,i.e; Velux, then NO not an option!, again as W/S staes need a pitch of a min 20°.. , however, if it is just puely light required, then yes, you can , but they will need an upstand with a minimum of 100mm , and "fillet pieces all round, and if you were thinking of felting the roof your self, unless you actually know what you are doing, then it can be a nightmare sorting the upstands corners!!
having said that , i did my sisiters kitchen extension many years ago now, (4 ally framed /georgian wired glass units-plenty of extra light!!) got a proper roofer to do the felt work, and it is still 100% water tight!!
they actually came with a sub frame which is screwed down to the upstand, felted upto and onto, then when the felt is done, the main glass/ally frame piece, clipped down , and hangs over like a square mushroom!!
and in regards to the flooring, .. the GREEN T & G chip board is a better bet, 18mm thick minimum!!! again due to weight loadings!!
and don't forget your electrics, you will need to think carefully about this! where the sockets are going , what sort of PLUGS!!!!!!

, you intend to use etc!!!!!