It's not exactly a derived crypto-currency, but it's an enabler technology that seems to be getting more integrated with Bitcoin.
Are questions on OT on topic here?
I would say occasionally
Using an analogous situation, if this site was about HTTP and someone was asking questions about HTML then the question would be clearly off-topic. HTML has little to do with HTTP other than using it as an efficient transport mechanism.
What is making OT appear on-topic is that it currently sees Bitcoin as being the most efficient method for value transfer. This gives rise to useful questions about how Bitcoin can better serve OT requirements (where appropriate) but care should be taken to avoid questions that are just about OT without a Bitcoin angle.
Drawing again from the HTTP/HTML analogy, a question about HTTP URI formation and it's use within HTML anchor tags would be relevant, but one about CSS styling would not.