Obviously all PCs joining Empire are taking the oath, but how about the general populace? Peasants, craftsmen? Are they all bloodoathed? Or only the fighting people (PCs, guildguards, centurions)?

I can sort of imagine everyone being required to take the oath once they come of age, but I want to know the "official" lore.

Let's say I'm formerly a peasant who worked those fields described in room descs north of Balator. Do I have to be orderly evil? Do I have to pick Imperial Reject flaw if I'm not going Empire?

To keep it as simple as possible, if your character was born/raised/lived in the Imperial Lands before beginning your adventuring career AND you took the late bloomer flaw, I would think for the sake of a consistent and "fully documented" development, it would make sense to be E/O as well as imperial reject. Key part is IF you took late bloomer. Otherwise, you could just spin the story you left the lands before the age of oathing etc.

If your character was born/raised/lived in Balator and just happened to work in this fields that are north of Balator (but not in the Imperial Lands) then you would really have no reason to be any sort of imperial loyalist or an imperial reject by default.

I assume this means that all adult imperial citizens, down to the last peasant, are oathed.