Today I stuck my head into the labyrinthine depths of DeviantArt. There I stumbled across a couple of free-as-in-beerdom tilesets of pretty good quality which look like they might be original work. However, the conditions are "no commercial use" which doesn't appear to be compatible with any of the licenses allowed on OGA. Also, I don't currently have a DA account in order to ask her if her work could be reposted here.

Here they are, if you want to check them out yourself:

https://www.deviantart.com/chaoticcherrycake/art/Free-Tileset-v0-1-8-828...

https://www.deviantart.com/chaoticcherrycake/art/Free-Tileset-16x16-8166...