In modern Japanese, the postpositional particle -o marks the accusative case on direct objects.
(1) Hanako-ga kuruma-o katta. Hanako-nom car-acc bought ‘Hanako bought a car.’
Aside from a handful of exceptions that are easily explained, the requirement in modern Japanese that the accusative marker -o must accompany a direct object is inviolable.2 As I showed in Miyagawa 1989, chap. 5, the accusative case marking is licensed by the case-assigning feature on the verb.