Where to start with Oracle?
Oracle's official documentation library has some great resources, including:


The Database Concepts guide, which is essential reading for beginner and Oracle ninja alike.
The 2 Day DBA
The 2 Day Developer's Guide


Oracle 12c official PL/SQL documentation:


PL/SQL Language Reference
PL/SQL Packages and Types Reference
Chapter 5 Developing Stored Subprograms and Packages from 2 Day Developer's Guide


PL/SQL 101

This is a twelve part series of articles on understanding and using PL/SQL. It is written by Steven Feuerstein and published in Oracle Magazine.


Building with Blocks
Controlling the Flow of Execution
Working with Strings
Working with Numbers in PL/SQL
Working with Dates in PL/SQL
Error Management
Working with Records
Working with Collections
Bulk Processing with BULK COLLECT and FORALL
The Data Dictionary: Make Views Work for You
Wrap Your Code in a Neat Package
Working with Cursors