YOu cannot tell by the on line seat map if the flight is oversold.
THe airline "holds back" many seats for assignment at the airport for such purposes as giving last minute frequent flyers a better selection.
If a lot of people book but don't bother to select seats, the flight can become oversold while seats are still showing available on the seat map.
On some but not all airlines, being able to select a seat lowers your chances of being bumped.