We know that the ancient Greeks worshiped the Olympian Gods and their descendants but what about the Titans? did the ancient Greeks ever worship the Titans such as Cronos, Okeanos, Rhea and Hyperion?
If not, why? where there any legends of Greeks been punished for worshiping the Titans?
If so, how did the Greeks worship them? did they build temples in their honour?
Yes, they did. At the very least Pausanias in "Description of Greece" VI, 20, 1 mentions sacrifice for Cronus done once a year on the head of a mountain Elathion in Elis. Sorry, I can't find a reference for English translation of this excerpt.
Talking about Rhea, the late cult of Rhea-Cybele is widely known.
UPD. Pausanias "Description of Greece" I, 18, 7 tells about the temple of Cronus and Rhea in Athens.
ἔστι δὲ ἀρχαῖα ἐν τῷ περιβόλῳ Ζεὺς χαλκοῦς καὶ ναὸς Κρόνου καὶ Ῥέας καὶ τέμενος Γῆς τὴν ἐπίκλησιν Ὀλυμπίας. ἐνταῦθα ὅσον ἐς πῆχυν τὸ ἔδαφος διέστηκε, καὶ λέγουσι μετὰ τὴν ἐπομβρίαν τὴν ἐπὶ Δευκαλίωνος συμβᾶσαν ὑπορρυῆναι ταύτῃ τὸ ὕδωρ, ἐσβάλλουσί τε ἐς αὐτὸ ἀνὰ πᾶν ἔτος ἄλφιτα πυρῶν μέλιτι μίξαντες.
Sorry, I'm only able to retranslate this piece into English from Russian translation.
In this area there are ancient works: the brass Zeus, the temple of Cronus and Rhea, and the sacred circle of Gaia known as 'Olympia'. Here, approximately half-yard long, the ground went asunder and, they say, after the deluge in times of Deucalion all the waters went hither. Because of that, every year they drop hither the wheat flour mixed with the honey.
This text so far is not the evidence that the temple of Cronus and Rhea really existed in times of Deucalion. Pausanias only says that it is situated near the cleft (as comments suggest, the latter does not exist anymore) which appeared in those old times.
Based on my answer on the Mythology stack...
There used to be a theory that the Titans were actually the gods of the inhabitants of (geographical) Greece before the Greeks invaded and took it over. The idea there is that the story of Titanomachy is actually an allegory for the Greek takeover of their modern homeland.
This theory was popular enough back when I was in school 3 decades ago that it was presented to me as fact by an anthropology professor. The best online reference to it I could find today is from an online copy of The Minoan-Mycenaean Religion and Its Survival in Greek Religion by Martin Persson Nilsson (1950).
According to Mr. Nilsson, the identity of those chthonic Titan-worshippers would be the Mycenaeans. One would imagine the Anatolians (eg Hittites) would be another possibility. Backing this up is the fact that some Titans are loosely associated with Asia Minor. Another good possibility is the original non-Indo-European inhabitants of the area (who we have little record of, aside from the Minoans and some other nearby islands).
I'm guessing this theory is not currently in favor though, based on its vintage and the trouble I had finding sources for it.
