At DroidKaigi Reject Conference, I have talked about Okio, specifically the internal implementation of Okio, with @stsn_jp talking about the internal implementation of OkHttp. To describe it (maybe too) simply, Okio is a library for I/O and OkHttp is a library for HTTP+HTTP/2 client.
Both libraries are for Java and Android and applications, and both are the ones of the most famous libraries, especially in Android development.
* square/okio: A modern I/O API for Java
https://github.com/square/okio
* square/okhttp: An HTTP+HTTP/2 client for Android and Java applications.
https://github.com/square/okhttp
* Droidcon Montreal Jake Wharton - A Few Ok Libraries - YouTube
https://www.youtube.com/watch?v=WvyScM_S88c
* DroidKaigi 2018 Reject Conference - connpass
https://connpass.com/event/73993/