Note: You can also run the same commands through sqlplus.
Who you create the objects as depends on how your script is written. If you have specified the schema name in the create table statement then you can run as either system or as a user that has DBA role.
create table scott.product (prod_id number(10));
will create a table in the user SCOTT's schema and will be owned by him. This command can be run either as system or as a user that has DBA role.
create table product(prod_id number(10));
will create a table in the current users schema named product. If this is run as system, the object will be owned by system. (BTW, usually not a good thing.)