Cloud used Omnislash on Sephiroth when they were inside that dark building (when the sparks were flying out of the swords every time Cloud's hit was blocked...you can identify that it's Omnislash due to that, and the fact that he's swinging extremely rapidly) and again, for fewer hits, about a minute later.

The final attack used by Cloud, to finish Sephiroth, was Cherry Blossom...you can see it if you get him in Final Fantasy Tactics.