The OEM-Unlock option is available with Android 5.0 “Lollipop” and higher, but not implemented in every Samsung device.
Especially older “SAMs” like the GT-I9300 don’t know this feature, because their last StockROM version was Android 4.3 “Jelly Bean”.
So in this case, forget the note to “OEM Unlock” and move on to the next step.