Is it possible to use an Oracle database for Drupal 8? We are evaluating Drupal 8, and one of our needs is to use Oracle as the main (and only) database.

Is it possible to use Oracle as the Drupal 8 database? Can be a problem with the external modules? 

And another question: The database access layer in Drupal 8 is Doctrine, or module developers can use hand-written queries (so, MySQL dependant queries)?
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