OPR is an acronym for OnPage Rank and is quite similar to the Google PageRank.
The OPR shows you how strong a site is linked within your complete website structure. The more incoming links a page has the higher it's OPR will be. Pages with low OPR are linked rather weak. Try to increase the OPR of your most important pages by adding incoming links and other references to these pages.
Find more information here: OnPageWiki OPR