They oversell on popular routes, i.e LHR-ARN. I spoke to some gate agents and on one day, a flight was oversold by 39 (late LHR departure) and the next day the number wasn't much better. Needless to say, if stuff like that worries you, use OLCI early and show up at the check-in in time to have the highest chance of being rebooked earlier. If it's full, it's probably overly full, and then it just comes down to airport, time of last flight, etc which will be the remaining factors.