First, try uninstalling and reinstalling Oxide From the "Non-Workshop Mods" section in your game panel. 

 

Next, try Force Validating your server Files:

Step 1: Click on the 'File Manager'.

Step 2: Check off and delete the 'steamapps' folder.

Step 3: Run the 'Steam update' script from the control panel.

Step 4: Files will validate.