better get training for its architectureself-learning may miss-lead you to nowhere. You need a solid training to get the basics like database architecture.
other areas can be self-learnt. Oracle has 5 fundamental parts: architecture, PL/SQL, performance tuning, backup&recovery, OEM/etc.
But..., recent job market for Oracle DBA in Singapore is not good, I have 10+ years working experience, not find a job yet!!!
Oracle is the most popular database AND dominate industry market up to 70% or more.
Big data is a new thing too young to make real product/profit.
Oracle has been taking the market in the past 15 years already; it keeps purchasing other companies like Sybase, Sun, Siebel, etc. Oracle has good market strategy and will keep its market in the next 10-15+ years, I feel.