W. M. Lindsay, Syntax of Plautus

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The Ablative with opus est is usually explained as a relic of the Instrumental Case, e.g. opus est gladio ‘there is a work (to be done) with a sword’¹

To the Agent the Genitive would be as appropriate as the Ablative to the Instrument, e.g. Most. 412 “id viri doctist opus”. We find the Genitive of the thing in Lucilius 334 Ma. “nummi opus” (see Marx's note). In Plautus we find also the Nominative, e.g. Capt. 164 “opus Turdetanis, opust Ficedulensibus, iam maritumi omnes milites opus sunt tibi”; but whether the Grammarian Nonius Marcellus (482 Me.) is right in saying that the Accusative was also used is doubtful. Cf.

Truc. 88 (of uncertain text),
• 902 “puero opust cibum” (-bo, edd.),
• Ter. Phorm. 666 “opus est sumptu (-tum A） ad nuptias.”

It might be defended on the analogy of usus est with Accusative, of which we have a probable example in Pseud. 385 “ad eam rem usust hominem astutum, doctum, cautum et callidum.” In this line the Verbal Noun usus seems to take the earlier construction (see above, 45） of the Verb utor (whence a use of the Gerundive like haec utenda sunt). Some however prefer to ascribe the Ablative with opus est to the analogy of the Ablative with usus est (the usual construction, just as utor in Plautus normally takes Ablative). Both usus est and opus est are found with Ablative of Perfect Participle Passive, e.g. Amph. 505 “citius quod non factost usus fit quam quod factost opus.” Cf. Pers. 584 “A. opusne est hac tibi empta? B. si tibi venisse est opus, mihi quoque empta est.”