Key calcium ion sensor involved in the Ca(2+)-triggered synaptic vesicle-plasma membrane fusion and in the control of neurotransmitter release at these output synapses. Interacts in a calcium-dependent manner to the presynaptic SNARE proteins at ribbon synapses of cochlear inner hair cells (IHCs) to trigger exocytosis of neurotransmitter. Also essential to synaptic exocytosis in immature outer hair cells (OHCs). May also play a role within the recycling of endosomes. Binds Ca(2+). The ions are bound to the C2 1 domain. Interacts with SNAP25; the interaction is direct. Interacts with STX1; the interaction is direct. Interacts with RAB8B. Detected at basolateral cell membrane with synaptic vesicles surrounding the ribbon and at the presynaptic plasma membrane in the inner hair cells (IHCs) at postnatal day 30 (P30). Colocalizes with GPR25 and RAB8B in inner hair cells. Additional isoforms seem to exist. Isoform 1 is expressed in cochlea and brain. Expressed in the cochlear and vestibular hair cells. Expressed in both inner and outer hair cells (IHCs and OHCs) and cochlear ganglions neurons at postnatal day 2 (P2) and 6 (P6). Expressed only in IHCs at postnatal day 60 (P60) (at protein level). Strongly expressed in brain and inner ear. In the inner ear, it is mainly expressed in the cochlear IHC and vestibular type I sensory hair cells. Weakly expressed in eye, heart, skeletal muscle, liver, kidney, lung and testis. Expressed in the organ of Corti in the inner hair cells (IHCs), but not in the outer hair cells (OHCs) at 16 dpc. Expressed strongly in the IHCs and faintly in the OHCs at 18 dpc (at protein level). The N-terminal first 124 residues can be classified as C2 domain, based on their 3D-structure. They are not sufficient for calcium ion or phospholipid binding (By similarity). Mice lacking Otof display hearing loss (PubMed:17055430, PubMed:17329413). Both outer hair cells (OHCs) and the afferent auditory pathway are functional (PubMed:17055430). Despite normal inner hair cells (IHCs) and ribbon synapse ultrastructures, these mice exhibit an almost complete abolition of IHC synaptic exocytosis in response to cell depolarization (PubMed:17055430). Belongs to the ferlin family.