Create the many-to-one relationship by using OEM to create a foreign key on the "many" table.
I don't think you can create a m:m relationships in Oracle. m:m relationships are really two m:1 relationships plus a join table (an assoiciative entity type, in ERD speak.) For example, the association of ADDRESSES to PEOPLE is m2m, so between the table ADDRESSES and the table PEOPLE you would create a table called ADDRESS_USERS. The relationship from ADDRESS_USERS to PEOPLE is m:1, and the relationship between ADDRESSES and ADDRESS_USERS is 1:m. When you look at it going from m:1 then 1:m, you can see quickly how it really supports m:m.