Occupying 3 oasis in all cities.

You can only occupy 3 oasis in a city that has a mansion in it. I recommend you do not build your main city right away. When you build a 2nd city make that your outpost so you are able to build fields higher then lvl 10. Build a mansion in your city to lvl 1. Build the career center to lvl 20. This will allow you to take 3 oasis. After you occupy 3 oasis destroy your mansion and move it to your next city and repeat. If your city does not have 3 oasis close enough to occupy them all then don't worry about having the mansion there.

Great advice, however if I may add something to what you are talking about. I recommend actually raising the level of your mansion in your 2nd city to lvl 10. This will allow you to produce more pioneers to take your 3rd city. Once you establish your 3rd city, then demo your mansion in 2nd city and reconstruct in 3rd. Seems like a lot of work but when you factor in the value you are adding, it is well worth it.