ό, τι means the same as οτιδήποτε (= anything, whatever thing )
For example: Πάρε ό,τι θέλεις. (=Take whatever you want.)
Θέλω ό,τι θες εσύ. (I want what/whatever you want.)
However, οτιδήποτε(which is the same) can be used alone as an answer, while ό, τι can not.
For example: Τι θέλει; (=What does he want?)
Οτιδήποτε. (= Anything, whatever thing)
But watch out, because ό,τι with comma is different from ότι (without a comma)
Ότι without comma means ''that'' .
For example: Πιστεύω ότι θα έρθει. (I believe that he will come.)
They are pronounced exactly the same ;)
Are you learning greek? I'm from Greece so if you need any help don't hesitate to say so! I'd love to help.