H2 and Oracle Compatiability issues
Short Answer
Oracle is a huge piece of software, and you can't expect a small embedded database to be 100% compatible.
Long Answer
Compatibility Modes
As documented, for certain features, this database can emulate the behavior of specific databases. However, only a small subset of the differences between databases are implemented in this way. This list is documented.
Compatibility
As documented, H2 is (up to a certain point) compatible to other databases such as HSQLDB, MySQL and PostgreSQL. There are certain areas where H2 is incompatible.
All database engines behave a little bit different. Where possible, H2 supports the ANSI SQL standard, and tries to be compatible to other databases. There are still differences however.