Just started using Oracle, no previous experience. I do have a good amount of experience with MySQL (and played a little with PostgreSQL). Could you recommend me a good introductory book to Oracle that takes into account the fact that I do have experience with these other DBs?
Anything that Tom Kyte says. He's written a few books for different purposes, so you'll want to pick the book specific to your concern. He's the most informative and helpful author I know of for Oracle.
For a stackoverflow-like experience with Oracle, you can also go to http://asktom.oracle.com.