With Microsoft Exchange Online via BPOS (the precursor to Office 365) you were able to configure a simple CNAME redirection to make access to OWA easier for your users.
For example, you could create a CNAME in DNS for mail.fabrikam.com (where fabrikam.com is your domain in BPOS) which pointed to go.domains.live.com and then when users accessed mail.fabrikam.com they would be redirected to the correct login page for BPOS. Note that this is also possible by setting the CNAME target to outlook.com.
A problem here though is that if you use go.domains.live.com then once you complete migration from BPOS to Office 365 the redirection stops working and when your users visit mail.fabrikam.com they get go.domains.live.com and not OWA!
So before you start your migration make sure you have changed your CNAME to outlook.com rather than go.domains.live.com and when users visit http://mail.fabrikam.com they get OWA both before and after the migration.
This recommendation also applies to academic organisations moving from Live@Edu to Office 365 for Education.