Currently, QuickBooks Online does not have a Company Setting to set a default bank account for writing checks. However, QuickBooks Online uses "sticky" settings. It will remember the bank account used in the last transaction of that type. Although this is not a default account, it works similarly for an account that you use regularly.
Features such as Checks, Pay Bills, and the Deposit to field on the Receive Payments screen will always populate with the last account used when a new transaction is entered.
Note: The "sticky" settings are user specific. Each user will default to the first bank account listed on the Chart of Accounts until they have created a check with a different account.