Hi Rajat

1. why not make oracle_account_number__c a formula field? like - Account__r.account_number__c (should it be editable?)

2.My basic rule is - if you are going to assign values to a record being insert/updated..., do it in a before trigger. what happens is that you "hitch a ride" on the already occuring DML (insert/update...) and assign the values you want.

so, if you do end up with a trigger, it should look like:

trigger IntegratePOtoAccount on Purchase_Order__c (before insert, before update) {

    //Create a set of account Ids
    set<Id> accIds = new set<Id>();
	
	List<Purchase_Order__c> pos = new List<Purchase_Order__c>();
 
    //Fetch the account id from Purchase Order
    for(Purchase_Order__c po : trigger.new)
	{
		//lets check if the record has an account id - 
		//this is in case it is not mandatory field.
		// if its not mandatory and you end up having null values in your query for accounts, 
		//you get really bad performance when scaling up + there is no account to take the value from, so why bother?
		if(po.Account__c != null)
		{
			//check if it is an insert - in which case we have to roll down the value for the first time (we already know we have an account value in the record
			//	OR
			//if its an update and the PO changed accounts - in which case we have to reflect the account_number__c of the new account
			if(trigger.isInsert || (trigger.isUpdate && po.Account__c != trigger.oldMap.get(po.Id).Account__c)
			{
				//add the record account__c value to a set in case we havent inserted it already while iterating another PO that was inserted/updated for that account
				if(!accIds.contains(po.Account__c))
				accIds.add(po.Account__c);
				
				//keep a list of purchase orders that passed the filter and therefore are relevant - this way you wont have to iterate over
				//the entire trigger list afterwords (good for governor limits and performance)
				pos.add(po);
			}
		}
	}
	
	//basically checks if we encountered POs that passed our filters - in which case we need to retreive their accounts and update them respectively
	if(!pos.isEmpty())
	{
		//Query for the relevant accounts. you dont need to query for every account in your org
		//use the below syntax to get back a map of account id to account object
		map<Id,Account> accs = new map<Id,Account>([Select Id,account_number__c from Account__c where Id IN: accIds]);
		
		//iterate the relevant purchase orders and assign the value of their repective account to oracle_account_number__c
		for(Purchase_Order__c po : pos)
			po.oracle_account_number__c = accs.get(po.Account__c).account_number__c;
		
	}
	
	
	//AND YOUR'E DONE
}

Regards

And please mark it as resolved if it solves you problem - could help other people encountering the same problem.