To find the last time a pure OG got drafted in the top 20 you have to go back to 2013 (lol), when Jonathan Cooper and Chance Warmack went at 7 and 10 respectively and Kyle Long went at 20. Though that includes Martin being drafted as a tackle with the intent to kick him inside. Before that the last time was Mike Iupati at 17 in 2010 and that was the first time since Leonard Davis, who went at 2 in 2001.
(Sidebar, from that year an OG went in the top 20 at least once every year dating back to 1992)
So as to Nelson going in the top 5, it would be unprecedented to the point that I'd suggest it hasn't happened in many members' lifetimes. Though it depends on if you count guys like Brandon Scherff. I'll let the good reader decide how often that happens because I can't be bothered going through all that.
http://www.nfl.com/draft/history/fulldraft?type=position&position=OL