ORCA is the low-energy branch of KM3NeT, the next-generation underwater Cherenkov neutrino detector currently being built in the Mediterranean Sea. The detector is optimised to study oscillations of atmospheric neutrinos in the few-GeV energy range in order to determine the neutrino mass ordering. Within its instrumented volume of more than 7 Mm$^3$ of sea water an unprecedented statistics of >3k $\nu_\tau$ neutrinos per year will be detected that have oscillated from a purely $\nu_\mu$ and $\nu_e$ initial flux into the $\nu_\tau$-channel along their passage through Earth. The $\nu_\tau$-flux will be determined as a statistical excess of shower-like events. The contribution will present the event classification strategies developed in ORCA and the sensitivity of ORCA to $\nu_\tau$-appearance. ORCA will confirm the exclusion of non-appearance within the first months of operation.