OTP in an Erlang context refers to the Open Telecom Platform - a bit of a misnomer since you can do a lot of things with it that do not inherently relate to telecoms, but that's how it evolved.
For context, Erlang itself was originally created at the telecom company Ericsson — the name is short for "Ericsson Language". But as concurrency and networking became bigger parts of software in general, the qualities that made Erlang good for telecom software turned out to be generally useful.
I think the name was a play on both "Ericsson Language" and the name of Agner Krarup Erlang, who pioneered queuing theory (before it was known as such) to address the fundamental question of inter-exchange capacity planning.
The unit "erlang", for "1 busy circuit/resource" is named in honor of that research.
