Systematic name of OCS (COS)
I'm not sure if there's definitive answer to not-yet-asked question, whether this compound can be considered organic or inorganic compound.

While organic nomenclature books do not mention it (acording to the current rules it might be called sulfanylidenemethanone, which is somewhat absurd), official IUPAC inorganic chemistry nomenclature (Red Book 2005, table IX, p. 292) lists two names for this compound:


carbonyl sulfide
oxidosulfidocarbon


Your mentioned (organic) systematic name thioxomethanone is just a variant of already mentioned sulfanylidenemethanone, as S= group's preferred name ("preselected prefix") is now sulfanylidene-* instead of thioxo- (rule P-15.5.3.3)



*) see analogy H2C=/H3C− methylidene-/methyl-, and
S=/HS− sulfanylidene-/sulfanyl- (older name: mercapto-)