Documentation for the OpO triple store database contains information about the HTTP API while the OpO-C documentation describes the client APIs. OpO use a JSON query language called TQL which provides options not found in other JSON databases.
A blog has been kept since the start of OpO. It includes rambling about what direction to take OpO as well as the latest news.
Benchmarks cover performance benchmarks for OpO as a simple web server, a database server, and as a web framework. In each case OpO performance is compared to the industry leader in that area. In each case Opo leaves the others far behind.