The answer is (as is so often the case) "it depends". If there is no foreign key constraint or triggers to enforce the parent/child relationship, then the user can absolutely delete the parent row.
If there is a foreign key constraint, the user MAY be prevented from doing so. However, there are ways to tell Oracle to DELETE all child rows or set all child rows to NULL first - which essentially kills or orphans all of the child rows so that deleting the parent row creates no conflict.
I recommend you read the Oracle Concepts Manual - in this case, the chapter on integrity constraints.