I've been trying to get oxide to work on my server, and it does download but it doesn't turn my server into a moded server and it also doesn't let me instal any mods. 

any ideas  

Sounds like you have something running SteamCMD to update the server on each start; you'd need to remove that. Oxide needs to be installed AFTER any and all updates.

I've always updated and then instaled oxide but it never works 

not sure if this is relevant but I always have this issue with the server saying something like ''main camera is missing'' and ''missing shader in effect''  

Hard to say without seeing your log file, but if you see no indication of Oxide, then Oxide was either not fully installed or was installed before SteamCMD ran.

Oxide is active. if you want I can copy and paste my log

What does the oxide.version command reply with?

Oxide.Rust Version: 2.0.4300

Could you paste your oxide/logs here using pastebin.com please?

Your Unity/Rust log from -logFile would also be beneficial.

According to your log, you have no plugins in the correct plugins folder. By default the oxide/plugins folder is located where RustDedicated.exe is unless otherwise changed by you or your host.