This has always been a pain for SharePoint Online administrators: setting up correct storage quotas when creating a new site (to get the longest time before being asked to extend) AND indeed extending the quotas when the site start reaching his limited storage.
This is (or will be soon) no longer a pain as Microsoft has now introduced (currently deploying across Office 365 tenant) a new feature called pooled storage model which in fact remove this storage quotas management by automatically adjusting the storage – off course up to the limit associated with your tenant (as reminder you get 10 GB + 500 MB * number of users). This feature does not apply to OneDrive for Business as this has already a unlimited storage.
The setting for this pooled storage will be set to true by default for new customers.
Existing customers need to go to the SharePoint Online Administration site and reach the Settings option to enable the feature.
Off course, you may still want to keep it disabled if you want to manage each site collection storage quotas.