It is possible but there is a round-about way of doing this.
If the proc. does not need any values:
1) Create a dummy table in Oracle.
2) Create an insert trigger on the table that fires off the stored procedure that you want to fire.
3) Create a linked server from SQL Server to Oracle.
4) Insert from T-SQL, a record in the Oracle table. Upon the insert, the stored procedure will fire.
I have tried this and it works fine.
Another alternative is to have a view in Oracle and create an Instead-Of trigger on it to fire off the procedure. From SQL Server, fire off an insert/update on that Oracle view which will fire off the trigger and thus the procedure and the old values will be present and can be used to pass in to the procedure. I haven't tried this one but it should work.
Hth