Tion kion mi ofte diras kiam mi eniras la ĉambron de miaj filinoj. Kio estis la bruo? Kio okazis? Kial estas via ĉambro tial malordiga?
Yes, okazi can never have a direct object son it's intransitive, which also means it can't have a predicate in a sentence.
"To be" and similar verbs are called stative verbs (verbs of state), as opposed to active verbs (verbs of action). Only active verbs can be transitive or intransitive. Transitivity doesn't apply to stative verbs. Also, the predicate is just the part of the sentence that isn't the subject. So after a stative verb is what's called a subject complement, which is often a predicate nominative or a predicate adjective.
"I don't remember anything! Who am I? Who are you? What is this place?"
"You can try all you want, but you're not getting out of studying, buster."
I got this right after "Kio okazas?" and for a second I thought it was the same sentence.