The position of soft obligations as legal sources is a continuously discussed topic in international law. As policy documents, guidelines and non-binding agreements often are regularly updated, the question of the applicability of different editions to various points in time may arise. The OECD Transfer Pricing Guidelines for Multinational Enterprises and Tax Administrations (OECD Guidelines) were revised most recently in July 2017, and these timing issues therefore appear in, for example, the undertaking of historical transfer pricing reviews and audits of years prior to the release of the updated OECD Guidelines. This article explores the arguments for using the new edition of the OECD Guidelines by comparison to an approach discussed in literature on finding guidance from different versions of the OECD Model Tax Convention (OECD Model) and its Commentary.