What does the "O" next to some of my subscribers name stand for?

The "O" stands for BrightTALK Organic, a free BrightTALK subscriber referral.

By participating as a public channel in the BrightTALK portal, you benefit from the work that BrightTALK and other channel owners have put in to growing the BrightTALK subscriber base. BrightTALK has over 5,000,000 users across various industries including technology, financial services, healthcare, marketing, legal, human resources and more. These people can access your content either through browsing our directory of live and upcoming webinars on the BrightTALK home page, via the community pages, or by searching for your webinars in the portal. BrightTALK referrals are free leads to add to your own database of customers and prospects.