We want to display  related lookup objects in a picklist when an opporunity is being created. In the opportinuty we have a lookup to brands. Brands are related to accounts. We want a pick list of brands when an account is selected when a new opportunity is being created.

I am using the standard opportunity controller with an extension. The extension can not use the account id on the opportunity untill the opportunity is saved. How can I reference the account on the opportunity?

Here is the logic that does not work: 

 public Test1_OppExtension(ApexPages.StandardController stdController) {
    this.o = (Opportunity)stdController.getRecord();

    }
 
    //builds a picklist of account names based on their account id
    public List<SelectOption> getBrands() {
 

        List<SelectOption> options = new List<SelectOption>();
         //new list for holding all of the picklist options
        options.add(new selectOption('', '- None -'));
 
        
        if(o.AccountId != null){
                 id account_id;
                 account_id = Opportunity.AccountId ;
                       
                for (Brand__c brand : [select b.id, b.name From Brand__c b where   b.Advertiser__c =: o.account_id ]){
                options.add(new selectOption(brand.id, brand.Name));
            }
        }
        
        return options;
     }