Composite Primary Key column order
Store the tenant_id first. When you do this you can enable index key compression.

See http://docs.oracle.com/cd/B28359_01/server.111/b28310/indexes003.htm#i1106790 for the syntax and http://docs.oracle.com/cd/B28359_01/server.111/b28318/schema.htm#i14618 for the concepts.

In your case, you can do it like this:

create unique index mytable_idx on mytable(tenant_id,id) compress 1;

alter table mytable add constraint mytable_pk primary key(tenant_id, id);