Nobody has really made this clear yet, so allow me:
Olympic bars are just short of 2″ diameter on the ends (actually 1 31/32″, or 50mm). As such, they will only take olympic weight plates, which have 2″ diameter holes.
Standard bars are 1″ in diameter on both the middle shaft and the ends. They are made to take Standard weight plates. You don’t want to put olympic weight plates on them because they will be sloppy and will swing around.