OPA is an onion router supplied as an Apache module. It is separate from, but may eventually interoperate with, the Tor network.
Traffic to and from OPA uses the PLATE protocol, which looks like regular GET and POST web requests. It is impossible to prove from the outside that anything special is going on besides regular web processing. A web server may even configure OPA as an add-on that functions in tandem with a vanilla page that is in use for other purposes.
OPA is by default configured not to be very active. By leaving it in its default configuration, a machine is not doing very much extra on the network, in memory, or on disk.
A route only exists for a single end-user request. There are no circuits that get re-used.
Node discovery works by a gossip protocol implemented by every node. By default, nodes are configured to lie a certain proportion of the time about other "known" OPA nodes.
This project is in a very early stage.