That being said, Octane is not actively developed anymore. Consider using Rattletrap (which is actively developed) instead. Rattletrap can also generate replay files from JSON, which Octane cannot do.
Octane has a command-line interface. To get it, download and unpack the latest release for your platform. You can run the executable one of three ways:
Pipe a replay file into it. It will output a compact JSON object to standard out.
$ octane < a.replay > replay.json
Pass it a path to a replay file. Both file paths and URLs work. It will output a compact JSON object to standard out.
$ octane a.replay > replay.json $ octane https://media.rocketleaguereplays.com/uploads/replay_files/9A06783F4FEA7AFF3D8298A3E5A412F5.replay > replay.json
Pass it several paths to replay files. Both file paths and URLs work. It will output a compact JSON array of objects to standard out.
$ octane first.replay second.replay > replays.json
Rocket League saves your replays in a folder that depends on your operating system.
%UserProfile%\Documents\My Games\Rocket League\TAGame\Demos
$HOME/Library/Application Support/Rocket League/TAGame/Demos
Octane is written in Haskell. If you're looking for a library written in another language, check out the Rocket League Replays wiki. It has links to many other Rocket League replay parsers.