First month is wherever you want for OE. As stated before.

Usually before you leave Dallas, there's a vacancy with numbers that the company chooses (per base). Within your class, based on DOB, the class bids for those openings. Obviously behind everyone already on the seniority list. So you're getting what's left over.

You select your bases in order you like them, and you get what you can hold. You can update your list whenever you'd like.