Line wrap. This image shows how to turn on/off line wrap in oXygen (Options > Preferences > Editor > Edit modes > Text):
A common error. The following validation error generally means there is an opening tag of some sort without a matching end tag, or vice versa (not necessarily <p>, as the error suggests):
Curly quotation marks. The XML editor will reject curly quotation marks enclosing attribute values. You may end up with these if you’re copying and pasting elements from anywhere into the editor. If you get an error, make sure you have straight quotation marks (just retype them in oXygen).