Is it possible to use an Oracle database for Drupal 8?
As of now no. only in Drupal 7 we have. 
Drupal 8 supports MySQL 5.0.15 or higher (or MySQL 5.1.30 or higher) with InnoDB as the primary storage engine, and requires the PDO database extension.
https://www.drupal.org/requirements/database 

https://www.drupal.org/project/oracle -  drupal 7 

see the here for further info the thread is postponed as of now

https://www.drupal.org/node/2037119