I'm inserting an Account, followed by an Opportunity.  Then, I run a SOQL to make sure the name field on Account is not null.  However, it keeps showing as null.  I know I can do a SOQL directly on Account, but this is just a simplified question for the real issue I'm trying to solve.  Does anyone know why this is showing as null?  Thanks in advance for your help!

Account a = new Account();
        a.Name = 'Universal Containers';
        a.BillingStreet = 'Market';
        a.ShippingStreet = '1 Market';
        a.BillingCity = 'San Francisco';
        a.ShippingCity = 'San francisco';
        a.BillingPostalCode = '94105';
        a.ShippingPostalCode = '94105';
        a.BillingState = 'California';
        a.ShippingState = 'California';
        a.BillingCountry = 'United States';
        a.ShippingCountry = 'United States';
insert a;

Opportunity o = new Opportunity();
       o.Name = 'Universal Containers';
       o.Account = a;
        o.CloseDate = Date.today();
        o.StageName = 'Submitted';
        insert o;
        
Opportunity o = [SELECT Id, Account.Name FROM Opportunity WHERE Id = :o.Id LIMIT 1];
System.Assert(o.Account.Name != null);
//This assert always fails even tough I know the Account and Opportunity were successfully inserted and the the Account.Name field is populated.