Is oracle difficult?
Oracle is no more complex than any other RDBMS, such as MySQL or
PostgreSQL. It has some unique features not found in other RDBMS',
and is missing some features that may be found in other systems.
Knowing "standard SQL" will give a basic level of knowledge needed
to use Oracle, and then, as with all other RDBMS software, some
additional learning will be required to take advantage of advanced
features.