Verbs conjugated in -a- (amō, amāre, amāvī, amātus), in -e- (habeō, habēre, habuī, habitus), and in -i- (audio, audīre, audīvī, auditus) are common and well-known. Verbs in -u- (acuō, acuere, acuī, acūtus) are less common and grouped in with the third (consonant) conjugation, but they definitely exist.
But are there any verbs in -o-? They're not common but certainly not unheard of in Greek (e.g. δηλόω dēlóō "show") and their complete absence seems odd in Latin. If there really aren't any, what happened to them?