I haven't got a copy of OpenOffice on any of my machines at the moment having moved on to the offshoot LibreOffice which is developing much faster. Here's some questions;
Do these grey marks print? Are they evident on other documents you open at the same time? If so I'm thinking aloud the lines that the document has the option to show non printing paragraph marks and formatting. On the toolbars look for the Backward "P" symbol likened by some to a musical clef. That toggles these on/off.