Starting Sept 2015, Oracle Standard Edition and Oracle Standard Edition One are no longer available from Oracle.  Oracle Standard Edition 2 is the new product offering under standard edition starting with Oracle 12 Release 1 (12.1.0.2) .

Below link from Oracle provides the difference between Oracle Standard Edition One (SE1) vs Oracle Standard Edition 2 vs Oracle Enterprise Edition:

https://docs.oracle.com/database/121/DBLIC/editions.htm#CIHBAEID

Additional references:

https://www.oracle.com/database/standard-edition-two/index.html

https://blogs.oracle.com/UPGRADE/entry/se2_some_questions_some_answers