Oasis

Is the limit per level defined per account, or per city?

-O

Per account.
Whatever city you occupy the oasis from will gain the resource boost.

Typically, most players use their main the most and house the largest portion of their army there since the main city cannot be taken via enemy attacks.

It's advisable to occupy all crop oasis' from your main city unless you are a notable city dropper that uses a sub city.

Yup.

The game design behind that is that players should make conscious decisions about which troops will be distributed across your different cities.

That said, I disagree Adina.

Unless you're at a constant threat from another player/alliance that are bent on stealing your sub cities, I would suggest you spread your army as evenly as possible across all your subs, and subsequently, capture oasis as needed per city. I say this because this way it will be FAR less likely to be starved for two reasons:
1 - Players will have a hard time identifying which oasis belongs to which city.
2 - Should your oasis be stolen, your negative crop sonsumption will be considerably less. Thus, buying you more time to log back in to replenish your crop stock.

Good times, right?

Perhaps.... but I prefer not spreading too thinly. I like having my masses but a minute away.
Having to arrange small masses and take portions of troops from up to 5 cities to pull them in to defend (to sucker punch those city droppers!) is just a right pain in the toosh!

For me, my way works and it works VERY well

That's the beauty of this game. To each his own.

Since I run a Speed/Cav army, travel times are not an issue for me. Even with my cities spread appart as they are, I can rally all my forces in under 2 minutes. I have never been starved, and that's not for lack of trying.