Oracle has a different syntax from MySQL for creating a function, but generally works the same way, as shown in the following query: 

CREATE FUNCTION hittinglevel(g_all IN NUMBER) 
     RETURN VARCHAR AS 
     hitlevel VARCHAR(10); 
BEGIN 
 hitlevel := CASE
 WHEN g_all BETWEEN 0 and 10 THEN 'barely any'
 WHEN g_all BETWEEN 11 and 50 THEN 'some'
 WHEN g_all BETWEEN 51 and 100 THEN 'many'
 ELSE 'tons'
 END;
 RETURN hitlevel; 
END; 

You can call the function and drop the function the same way that you do in MySQL.