Complex sentence: He succeeded in only a 15 second gain on Merckx at that stage but he built on that the following day and then on stage 11 Ocaña rode himself into the yellow jersey with over 8 minutes over Merckx.
Split into simpler sentences: He succeeded in a 15-second gain on Merckx but built on that the following day. Then on stage 11 Ocaña rode himself into the yellow jersey with eight minutes over Merckx.