I tried both and found the UI to be better in Roam. The WYSIWYG aspect of Roam is real nice, vs having unrendered markdown in one pane and the rendered version on the other side.

Also, the “daily notes” feature in Roam is really useful, it’s a small UI thing but it gives me a nudge to make a daily note when I do any thinking, rather than having to file it manually in Obsidian.

Obsidian has most of the core stuff that Roam does though.

Block Level reference is a huge advantage. Obsidian supports page based reference.

For starters roam is web based and the other one is desktop software. Also don't think there is a mobile app for later iirc. Also I was wrong that it's OSS. it's a free alternative, it's not OSS.