Miss.B, I usually O around CD17 and I always start the OPKs at CD10 in case I O early. The one time I decided to wait and start CD13 I actually O'd CD13 (and obviously missed the surge, just like you were saying). I had NEVER O'd that early in a natural cycle before, so it really caught me by surprise! Thankfully we were TTA that cycle (due to a skin cancer treatment incompatible with TTC), so it worked out okay.