I just learned (from Pé de Leão here) that in Neo-Latin orthography, Latin words that come from Greek names that end in -οῦς get a circumflex in imitation of the Greek circumflex, e.g. Trapezûs, Hierichûs. See, for example, p. 16 of this summary of Mindaugas Strockis's doctoral dissertation, Accent Notation in the Classical Languages and its Influence on Lithuanian Accent Notation.
So, does octopus get a circumflex?