If a file marked as read-only is opened in Oxygen XML Developer you can by default perform modifications to it. This behavior is controlled by the Can edit read only files option. When attempting to save such files you will be prompted to save them to another location.
You can check out the read-only state of the file by looking in the Properties view. If you modify the file properties from the operating system and the file becomes writable, you are able to modify it on the spot without having to reopen it.
The read-only state is marked with a lock decoration which appears in the editor tab and specified in the tooltip for a certain tab.