This quickly gets into the details of definitions, but I think by most people's definitions of 'open source', something that is 'public domain' qualifies as such (see also 'source available' or 'copyleft/free software', one of which is not quite open source and the other is a more restrictive kind of open source. 'permissive' licenses like MIT and similar are closer to public domain but are different to varying degrees of technicality: one of the main problems with 'public domain' is that it's not universally accepted that there's any means to deliberately place a copyrightable work into it, so something like sqlite where the authors are not long dead is not actually public domain according to many jusrisdictions)