Hi there 

What I did was
-Omega Theme Install
-USE Admin Themes Plugin-Configure Change Omega Theme
-USE Side Menu-Alpha Osclass Theme Configure

--The following error appears at the top of the Admin Panel
Warning: file_get_contents (/home/***/***/public_html/oc-content/plugins/oc-content/index.php): failed to open stream: No such file or directory in / home / *** / *** / public_html / oc-includes / osclass / classes / Plugins.php on line 357

Please let me know the solution.

Regards

@SKson
Error does not reference to theme.

@Support team

This error will not appear for standard admin panel themes.
-Modern by Osclass: Default osclass oc-admin theme

I have not customized Omega Admin Theme 1.0.1 by MB Themes.

I do not know what is the cause...

If it does not show, it does not mean it is not there. You have answer to your question clearly explained in error message