There are possibly two types of information that you would like to move to Oracle

1. User information Stored in Active directory
2. User Profile information Stored in Sharepoint

for (1.) Information stored in Active Directory, there is a pretty standard API available, (I donot have time to paste the code  here, but you will find it easily if you google a little)

for (2.) if you need to move the User Profiles stored in Sharepoint (if some new fields have been added updated over and above the information from the Active directory), then only you need to access the Sharepoint Object Model,

You will need to access a List called "User Infomation List" where the Users are stored by default, enumerate all the Items and store them to your Oracle Database.
( Again I dont have time to paste the code here but.. to give you a hint, Classes that you will use are - SPsite, SPweb, SPlist, SPitem... google on them)

Haven't much tried them but try using Sharepoint Web services... might speecd up the process

i used sharepoint  webservice to captured properties of current user  logged in site  
why u want to connect AD as all the properties defined in AD can be imported into sharepoint userprofile and it is simple to retive from user and profile
u will have to add namespace microsoft.sharepoint.userprofile

Check out

http://moss-solutions.blogspot.com/2008/10/iterating-userprofiles-in-shared.html