In your campaign settings, you have the ability to set a minimum threshold. Below it Octopush will send you an alert.
You can receive this alert on your account email, or by SMS to the number of your choice.
By setting a large enough alert level, you will have time to come back to the platform to order your credits before it reaches 0. In which case this would cause a possible service interruption.
Also, you can use the Octopush API feedback yourself, which tells you your remaining credit, to trigger an alert.