H
"ότι" means "that". Example: Έμαθα ότι έπιασες δουλειά. (I've heard that you got a job.)
"ό,τι" is one word and means "whatever" or "everything". Example: Πάρε ό,τι θέλεις. (Take whatever you like.)
Therefore "Θέλω να ξέρω ό,τι ξέρεις" means : "I want to know everything that you know."
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.