To get the answer this question, you'll have to wait for a Forum member who knows both SQL Server and Oracle. However, if you are in a hurry, you could explain what this script does. OK, it is obvious that creates a table, but - what is "identity(1, 1)", what does "ignore_dup_key" do, what is "on [primary]", etc.
If the answer is (not necessarily, but could be) "Google for it and you'll know", well, you could also Google for Oracle's CREATE TABLE script
Now seriously, just as a first aid: BIGINT is, probably, a datatype. I have no idea what IDENTITY means, so I'll leave it alone. Constraint is OK, while ASC might mean that this column autoincrements in ascending order and you don't want to allow duplicates to appear.
If so, here it is: as Oracle doesn't support autoincrement columns, we'll have to simulate it using a sequence and a database trigger:
SQL> CREATE TABLE user_authorization
2 (member_id NUMBER PRIMARY KEY,
3 other_columns NUMBER
SQL> CREATE SEQUENCE seq_auth;
SQL> CREATE OR REPLACE TRIGGER trg_auth
2 BEFORE INSERT ON user_authorization
3 FOR EACH ROW
5 SELECT seq_auth.NEXTVAL INTO :NEW.member_id FROM dual;
As an illustration, let's add a few records and see how it works: