I wouldnt class NWA as Old School Myself but i guess it depends on how long you have been listening to it! I agree with Sunnyday the Electro series (sadly only ever officially released on Vinyl or Cassette although some sellers on ebay have converted them to Cd but i have no idea what the quality is like) is a must for any diehard fan and you can then see how it started to all come about and take shape, I would also recommend Afrika Bambaataa & the Soulsonic Force, Eric B & Rakim , Run Dmc, Mantronix, Biz Markie, BDP, Public Enemy , ICE T, LL COOL J when he was still good! Dont forget too Hijack probably the best and most underated and most credible UK Hip Hop outfit to ever come out of the UK although their album The Horns Of Jericho is very hard to come by unless you wanna pay a lot of money for it, Silver Bullet had some good tunes too. Anyway theres a lot out there and probably loads that i have missed but well worth a listen. I hope you find some stuff that you like, good luck searching for it!