ό,τι means "whatever", "no matter what"
Example: Ρώτησέ με ό,τι θέλεις. = Ask me whatever you want.
Ό,τι και να πω, λέει το αντίθετο. = No matter what I say, he says the opposite.
ότι means "that".
Example: Μου είπε οτι θα έρθει αύριο.= He told me that he will come tomorrow.