Osprey does come with a few pouches. I prefer chest rigs as you can carry more mags, a water bottle, grenades and a little bit of other sh1te. Throwing everything in a day sack is all well and good until you have to leave it somewhere and don't see it again.
Actually, there is a chest rig on the market with extended shoulder and waist straps to accomadate osprey, as well as an assault vest that will fit over same. But most guys seem to be buying leg mounted pouches as they are easier for vehicle ops, and not so 'torso-bulky'. The company that makes them also sponsors the infantry forum, if anyone is interested.
The ammo pouches that come with Ospret have poor fasteners. They are the same pattern as PLCE but smaller and fiddlier. They are rather more likely to come undone than you would like; it is worth getting some chunky velcro sewn onto the inside of the pouches so they can bge easily ( and securely ) fastened.
I've also seen a fair few blokes about to go to Afghanistan buy magazine drop pouches; these are seen fastened to the upper left thigh, the idea being that its quicker to put an empty mag into this than back into a webbing / Osprey pouch.
try, using the molle pouches made by blackhawk, you can get the pouches you need to suit the op, i used the tripple m4 pouch, this fits two IW mags each, the frag pouchs are execlent for the L109 and there is even a pouch for the red phos, all there fit to osprey, the raido puch will fit PMR.
The viper vest is also a good bit o kit and will fit over osprey it comes with lace witch you adjust to suit yer build, fits the standard mag, no gren pouches though.