There's perhaps something of hint of a meta-joke lurking in this demand for the original author of "otherness". Very generally speaking, the idea of the other (sometimes called alterity) is:
(a) phenomenologically disruptive of our self-assurance and authority to "legislate" our own words/writing (Derrida, Levinas), and
(b) epistemologically confounding, through presenting us with fundamental un-knowability, at least as to the inner sense of the enjoyment/desire of the other (Lacan, Zizek, but Nietzsche belongs here too: what if nature/wisdom were a woman?)
Hence altogether then we have a map of a kind of onto-epistemic humility -- and the traces here of so-called negative theology can be detected.
Heidegger and Levinas are perhaps close to an originary dialectical couplet here of the modern conception of alterity and otherness, who between them articulate alterity in its horror and intimacy. It is Levinas who recaptures a transcendental sense of phenomenological alterity in the infinity of the face-to-face encounter and the subsequent demand of the other upon us, which at its most fundamental is simply to let the other live: the most basic sense of alterity in this sense is an ethical injunction not to murder.