trigger InsertingBU on Opportunity (after insert,after update) 
{
	if(Trigger.isAfter && Trigger.isInsert) {
		opportunityHelper.oppHelperMethod(Trigger.New);
	}
	
	if(Trigger.isAfter && Trigger.isUpdate) {
		opportunityHelper.oppHelperMethod(Trigger.New);
	}
}

public class opportunityHelper{

 public static void oppHelperMethod( List<Opportunity> lstOpp){
	Set<Id> accountIds = new Set<Id>();
    for(Opportunity o : lstOpp)
        accountIds.add(o.AccountId);
        
    List<Business_Unit_Company__c> buCompaniesToBeUpserted = new List<Business_Unit_Company__c>();   

    Map<Id,Business_Unit_Company__c> existingBuCompanies = new Map<Id,Business_unit_Company__c>();
    
    for(Business_Unit_Company__c bu : [Select Business_Unit__c,Company__c,Stage__c from Business_Unit_Company__c where Company__c IN: accountIds])
      existingBuCompanies.put(bu.Business_Unit__c, bu);
      
    for(Opportunity o : lstOpp) 
    {
      if(o.Opportunity_Business_Unit__c != null && existingBuCompanies.get(o.Opportunity_Business_Unit__c) != null)
      {
        Business_Unit_Company__c relatedBuCompany = existingBuCompanies.get(o.Opportunity_Business_Unit__c);                              
        if(relatedBuCompany.Stage__c != 'Closed Won')
        {
          if(o.StageName == 'Closed Won')
          {
            relatedBuCompany.Stage__c = 'Closed Won';
            buCompaniesToBeUpserted.add(relatedBuCompany);
          }
          else if(relatedBuCompany.Stage__c != 'Closed Lost' && o.StageName == 'Closed Lost')
          {
            relatedBuCompany.Stage__c = 'Closed Lost';
            buCompaniesToBeUpserted.add(relatedBuCompany);
          }
          else if(relatedBuCompany.Stage__c != 'Open')
          {
            relatedBuCompany.Stage__c = 'Open';
            buCompaniesToBeUpserted.add(relatedBuCompany);
          }
        }        
      } 
      else if(o.Opportunity_Business_Unit__c != null)
      {
        Business_Unit_Company__c newBuCompany = new Business_Unit_Company__c();
        newBuCompany.Company__c = o.AccountId;
        newBuCompany.Business_Unit__c = o.Opportunity_Business_Unit__c;
        if(o.StageName.contains('Closed'))
        {
          newBuCompany.stage__c = o.StageName ;
        }
        else
        {
          newBuCompany.stage__c ='Open';
        }
        buCompaniesToBeUpserted.add(newBuCompany);
      }   
    }
    if(!buCompaniesToBeUpserted.IsEmpty())
      upsert buCompaniesToBeUpserted;
 }
 
}