My trigger is working as desired however, I have issues with not enough test coverage.  I would appreciate some suggestions on how to test some of this code.  The lines NOT COVERED are those in Red and large font below.  I am really unsure how to procedd with testing these lines of code???

trigger triggerWorkorderOppUpdate on Work_Order__c (after insert, after update, after delete) {

    //************************************************
    // Build a LIST of Opportunity ID's that will
    // need recalculating
    //************************************************
	set <id> oppIDs = new set <id>();
	list <Opportunity> opportunity = [SELECT ID FROM Opportunity WHERE ID in:oppIDs];
	
    if(Trigger.isInsert || Trigger.isUpdate){
        for(Work_Order__c w : trigger.new){
            if(w.Opportunity__c != null){
          if(!oppIDs.contains(w.Opportunity__c)) 
                oppIDs.add(w.Opportunity__c);
            }

	        }
}
      // INSERT/UPDATE Trigger

    if(Trigger.isDelete || Trigger.isUpdate){
        for(Work_Order__c w : trigger.old){
            if(w.Opportunity__c != null){
               if(!oppIDs.contains(w.Opportunity__c)) 
                oppIDs.add(w.Opportunity__c);
            }
            //           w = [select ID, Opportunity__c from Work_Order__c where Opportunity__c in :oppIds];
        }
    }  // DELETE/UPDATE Trigger

    if(oppIDs .size() > 0) {

        Map<ID, Opportunity> opps = new Map<ID, Opportunity>([Select id, Estimated_Cost__c from Opportunity Where Id in :oppIds]);  
     
      	Opportunity d = null;

        for (AggregateResult dr : [SELECT Opportunity__c, SUM(Cost_WO__c) SubPO FROM Work_Order__c WHERE Opportunity__c in :oppIds GROUP BY Opportunity__c]) {
				
            String dID = (String)dr.get('Opportunity__c');
            //get record or create one
            
            if(opps.get(dID) == null)
            d = new Opportunity (ID=dID,
            	Estimated_Cost__c = 0);
        		
        	else
        	if(opps.containskey(dID))
        		d = opps.get(dID);
        		
            		
            // update the summary total fields on the opportunity
            Decimal po = (Decimal)dr.Get('SubPO');
                    
        	d.Estimated_Cost__c = po;
        	        	
        	opps.put(dID, d);
        }
        //commit the changes to Salesforce
        
        update opps.values();

    }

}

Test Code

public with sharing class testTriggerWorkorderOppUpdate {



    static testMethod void myTest() {

       //Insert a test opportunity
		Account a = [select Id, account.OwnerId, account.Name from Account limit 1];
		system.debug('Account' + Account.ID);
		
       Opportunity o = new Opportunity();
       o.OwnerID = a.OwnerId;
       o.AccountId = a.id;
       o.StageName='Target';
       o.Name='TestOp';
       o.closeDate=Date.today();

       insert o;
       system.debug('opportunity id' + Opportunity.ID);
       
         //Insert a Work_Order

       Work_Order__c wo = new Work_Order__c();

       wo.Opportunity__c = o.id;
       
       wo.Close_Date_WO__c = Date.today();

       wo.Cost_WO__c = 55.00;

 
       insert wo;
       
       wo.Cost_WO__c = 75.00;
       update wo;
       
		system.debug('workorder' + wo.ID);
		
       Opportunity updated_Opportunity = [SELECT ID, Estimated_Cost__c FROM Opportunity WHERE Id = :o.Id];

       //Verify that the values of the opp were changed by the trigger
       
  System.assertEquals(wo.Cost_WO__c,[Select Estimated_Cost__c From Opportunity Where Id = :o.Id].Estimated_Cost__c);
       
 
     }
static testMethod void testBulkInsert() {

  List<Opportunity> opps = new List<Opportunity>();
  
 }
}