Late last year, Microsoft announced that it would reduce free OneDrive storage from 15GB to 5GB and discontinue unlimited storage for Office 365 users. Microsoft's reasoning behind the move was that users were abusing the unlimited storage, storing up to 75TB in a single account. After user backlash, Microsoft reversed its decision, letting users opt-in to keep the 15GB storage.

In addition, Microsoft stated that users that have over 5GB of content stored on the service will be eligible to receive a one-year subscription to Office 365 Personal for free. For those waiting for the promotion, it looks like Microsoft is sending out emails through which users can claim the offer:

As you know, the amount of storage that comes with most OneDrive accounts is changing from 15 GB to 5 GB. You previously confirmed your desire to keep your 15 GB of free storage (and the 15 GB camera roll bonus if you have it).* As a result, your account will not be affected by these changes.

We also want to extend an offer that we are making available to users who are close to or will be over the new storage limits. You can claim a free one-year subscription to Office 365 Personal.** This subscription includes 1 TB of OneDrive storage. We want to apologize for any inconvenience these changes may have caused you. We listened to our users' feedback, and we are committed to making OneDrive the best option for people who want to be productive and do more.

Thank you for using OneDrive. – The OneDrive Team

According to the discussion on Reddit, you'll need a valid credit card to avail the offer. If you're not interested in renewing Office 365 after one year, be sure to turn off auto-renewal.

Anyone else received the email from Microsoft yet?

Thanks str8jacket for the tip!