I'm a little bit in the opposite boat. I've worked with oracle for 6 years and now use MS SQL server and t-sql.
I think we could help you better, however, if you explain (much like someone asked in a prior reply) just WHAT IS IT you're trying to do in Oracle? Oracle is HUGE to say the least and you could very well get two people that use Oracle and do very different things with it.
However, I suppose I can help assuming that you're interested in either a DBA role or PL/SQL (PL = Procedural Language) developer role.
Regardless of role, you should purchase the latest Oracle DBA handbook from Oracle Press. It's a bit big and intimidating and probably not cheap (I'm guessing between $60 - $80) but I think it's a superb reference for DBAs and PL/SQL developers.
You may also want to consider getting an "Oracle for Dummies" book. This book focuses on Oracle and presumes you have little or no prior DB maintenance experience. You'll quickly find that your MS SQL knowledge will come in handy.
If you will be in a DBA role only, then those two books should be enough to get you started. Please also keep in mind that you can download almost ALL Oracle DB software and play around with it yourself for FREE. Just join OTN (Oracle Tech Net) and agree to the terms that you will not develop commercial applications for sale with the software (i.e. you will be strictly a developer) and you can d/l it and have your own environment to set up in and play around in.
If you need to write PL/SQL then I would strongly suggest books from O'Rielly publishing titled:
Oracle PL/SQL Programming (by Steven Feuerstein (the ultimate Oracle PL/SQL guru))
and Oracle Built-In Packages (By SF also)