SQL & PL/SQL

Hello, I am very new to the Oracle DB as I have been working with MS SQL Server for many years. I have a SQL statement that works fine in MSSQL but seems to not work in Oracle. Is this a syntax problem or do I need to look elsewhere?

DECLARE @as_master_key varchar(26)
EXEC [dbo].[GET_UNIQUE_MASTER_KEY]
@as_project_name = N'CMDEMO',
@as_database_site = N'CMPL',
@as_user_initials = N'SK',
@as_table = N'SUBMITTAL_CATEGORY',
@as_master_key =
@as_master_key
OUTPUT SELECT @as_master_key as N'master_key'

This is what I use in MSSQL and it works fine - but I am porting this application to an Oracle backend and it fails. Any suggestions?

Thanks in advance to this newbie. :)