a few random thoughts on how we support oracle as well as other databases:
as roland said, think of a schema as similar to a sybase database. Create a schema owner that has all your database objects. Only use that schema login to change those objects.
those objects must all be prefixed with the schema owner name OR an alias must be provided OR the easy way is to set the user default schema: after login,
alter session set current_schema =
you have to grant access to all database objects. create role(s) that you are granting access to and then give users that role.
in oracle an empty string '' will be saved as a NULL and not an empty string in your varchar2 columns.
you can write stored functions in oracle to add sybase functions. for example you can write oracle functions for getdate(), left, right, substring, len, ...