Insert in child table prevents lock on parent table
The first statement,

insert into TBL_ActionHistory(ActionHistoryID, Message) 
    values (201, 'Something'); 


puts TM locks on TBL_TASK and TBL_ACTIONHISTORY tables to enforce FK constraint. 
The second DELETE successfully locks row in TBL_ORDERLINE, but it also has to lock the whole TBL_TASK table (because you don't have index on TBL_TASK.OrderLineID) .  From docs : 


  Locks and Unindexed Foreign Keys When both of the following conditions
  are true, the database acquires a full table lock on the child table:
  
  No index exists on the foreign key column of the child table.
  
  A session modifies a primary key in the parent table (for example,
  deletes a row or modifies primary key attributes) or merges rows into
  the parent table. Inserts into the parent table do not acquire table
  locks on the child table.
  
  Suppose that hr.departments table is a parent of hr.employees, which
  contains the unindexed foreign key department_id. Figure 9-3 shows a
  session modifying the primary key attributes of department 60 in the
  departments table.