FOSS is free, OSS doesn’t have to be. Very often open source software, of which the commercial fork is being maintained by a company, that company will profit from businesses using the software. Idk about VLC but Moodle, for instance, is open source and updates for it are based on a subscription model.
The license agreement for OSS will often state that you are free to use it in your own home, but if you start commercially using the software, they expect you to pay. Some open source projects can get resold by service providers this way to handle deployment of updates, provide support, et cetera.