I purchased the Oxygen template about a year or ago but do not have the details any longer. I have been using it successfully on a Joomla website (http://www.nodular-prurigo.org.uk) since. Recently I have had problems in saving new articles. On trying to save I get a 500 & 404 error. Examining the logs there is output such as

[Tue Oct 08 11:05:10 2013] [error] [client 180.245.54.91] File does not exist: /home/nodularp/public_html/jm/templates/j51_oxygen/css/menus.css, referer: http://www.nodular-prurigo.org.uk/jm/templates/j51_oxygen/css/template.css

Looking at /templates/j51_oxygen/css/template.css I note that menus.css is called at the beginning of the file template.css. Is menus.css part of the Oxygen theme as I seem unable to find it even in the original package? This problem is causing me difficulties in upgrading the site and although I am unsure whether this a problem of your theme or core Joomla functions, any help you can give would be much appreciated.