[roland-s]

If you're using table aliases like "O" you're already throwing clarity out the window. The following is perfectly clear and readable IMO:

    Orders.id = Customers.order_id

and also indicates which column is the primary key and which is the foreign key. What about other column names duplicated across tables, should

    Orders.created_at = Customers.created_at

become

    Orders.order_created_at = Customers.customer_created_at

Scope exists for a reason, don't be afraid to use it!

[Ma8ee]

The reason we use aliases is it almost only in toy-examples that most tables are called things like Order. I won't fill my SELECT with row after row with CustomerOrderRowHistory...

And your second example kind of prove my point. When it it written out like that it become obvious that you are trying to compare diffent quantities. There is clearly something wrong when you are trying to join on order_created_at = customer_created_at which isn't at all as clear with your way of writing.