Download a copy of Oracle and play with it. It is relatively easy (not exactly simple, but doable) to get running and connect to. The basic ansi commands (select update insert delete) are there, but aggregates and functions are different. Understand that oracle is far more complex than SQL Server. Doing a non standard, feature rich install is not for the faint of heart, and rolling that out to production can be amazingly difficult.

One the big reasons companies use it is it's not limited to windows. It has a history of large deployments, it's backed by a large company, and Oracle skilled are available.