It's worth noting that every ancient society established contracts essentially orally, whether or not they were literate. Roman law in particular, despite having a complex and often obtuse system of contracts and contractual obligations, did not have written contracts. Mutual agreements resulting in changes of legal definition were oral, and are treated as such in the many dozens of cases where they are defined in the Digest. To quote the New Pauly, since I don't think I need to reinvent the wheel here: "In the 2nd cent. AD, the jurist Gaius [2] (Gai. Inst. 3,89) differentiated four types of contract: they are created by agreement ( consensus), making a payment (a real contract, such as a loan; mutuum), the use of verbal formulas (verbis, especially the stipulatio) or the rather rare written contract ( litterarum obligatio)." Contracts established by agreement, which were not necessarily explicitly legally stated (though only a certain body of recognized types of agreements were actionable, and verbal agreement in general was not actionable) and did not necessarily require witnesses or oaths, the Digest defines as being regulated simply by bona fides. Most formal contracts, such as the transfer of res mancipi, required complex formulaic rituals, featuring oaths and multiple witnesses. These complex, and often highly archaic, formulas were increasingly done away with over time, but nonetheless the regulation of sworn contracts by rite and witness remains a typical feature of ancient societies. There's nothing particularly unusual about non-literate societies in this regard.