Hi HelloSan,

If you want the name of the Account Owner on the Opportunity, you can acheive this by simply creating a formula field of type Text and the fomula will be Account.Owner.FirstName + ' ' +  Account.Owner.LastName . This formula will work for both existing records and new records without any extra efforts.

You can achieve this by using by WorkFlow and triggers as well. But for both Workflows and triggers to fire there should be DML operation on those records. For newly created or updated records it will work But to populate that field for exixting records you have to update thos erecords.

You can simply update existing opportunities by just including the Id field in the CSV file and do update operation using data loader

For Workflow,
Evaluation Criteria will be every time record created or updated
Rule Criteria will be AccountId != null
Add a field update with formula Account.Owner.FirstName + ' ' +  Account.Owner.LastName 

If you want it will be a lookup field or if you want to do it by trigger below is the code
If you want a look up field on Opportunity lets suppose API Name will be Account_owner_lookUp__c
If you want a text field on Opportunity with name of account owner lets suppose API Name will be Account_owner_Name_Text__c
 

trigger UpdateAccountOwner on Opportunity(before insert,before update){
	Set<Id> accIds = new Set<Id>();
	Map<Id,Account> mapIdWithAccount = new Map<Id,Account>();
	
	for(Opportunity opp : trigger.new){
		if(opp.AccountId != null){
			accIds.add(opp.AccountId);
		}
	}
	if(!accIds.isEmpty()){
		for(Account acc : [SELECT Id, OwnerId,Owner.Name FROM Account WHERE Id IN :accIds]){
			mapIdWithAccount.put(acc.Id,acc);
		}
		
		for(Opportunity opp : trigger.new){
			if(opp.AccountId != null && mapIdWithAccount != null && mapIdWithAccount.get(opp.AccountId) != null){
					opp.Account_owner_lookUp__c = mapIdWithAccount.get(opp.AccountId).OwnerId; //For Lookup field
					opp.Account_owner_Name_Text__c = mapIdWithAccount.get(opp.AccountId).Owner.Name; //For text field
			}
		}
	}
}