To begin with you must have installed oxide from the control panel> mods & plugins (or manually uploading the files oxide by FTP).
If you opt for a manual installation you can download oxide here (requires a connection via steam).
Once oxide installed (if you installed it manually you will need to restart the server) you should see a plugins folder in the oxide folder ( \oxide\plugins )
All you have to do is download a plugin for oxide on this page then upload it by FTP (or by the file manager of the control panel) in your \oxide\plugins folder.
Then restart the server, if the plugin loads properly it will generate its configuration file in the \oxide\config folder.
If the plugin does not seem to load it is likely that either oxide is not up to date, either the plugin is not up to date or the plugin version is not compatible with your version of oxide.