The border between Oman and the UAE was finalised only as recently as 2008. This paper asks: over the span of five decades, what work has gone into making this border? How have local and foreign powers exploited legal and political disputes in the Arabian Peninsula littoral in order to push for delimitation in unmarked territory? And how has the border shaped the contemporary political landscape? Drawing on archival research in the UK, and 16 months of fieldwork in Oman between 2017 and 2020, this paper documents the cunning planning, the difficult demarcation and the constant contestation of what would become the Omani-Emirati border. The paper consists of two parts. First, it produces the genealogy of the border: its conception in telegraph exchanges between British diplomats stationed in the Arabian Peninsula, its necessity for obtaining oil concessions, and its imagined role in maintaining regional stability after Her Majesty’s Government’s (HMG) departure. It then traces how HMG capitalised on its role as arbiter in minor intertribal conflicts in order to force Oman’s Sultan Sa’id bin Taimur to commit to delimitation. Whereas some scholars (Beasant 2002; Valeri 2009) have asserted that Sultan Sa’id’s opposition to rapid oil-rent funded development in the 1960s occasioned the British-backed coup against him, I argue that Sa’id’s contrary vision of sovereignty in the 1950s already accounts for HMG’s gambits to subvert his rule. In the second part, this paper explores how the border exacerbates contemporary issues of national belonging and citizenship among border-dwelling Omanis. With Beaugrand’s (2018:223) provocative claim in mind that “statelessness is part and parcel of the modern Gulf state,” it draws a parallel between the bidun she studied in Kuwait and young Omani men living in bordertowns. Although they are, of course, included in the Oman’s social contract, many borderland Omanis feel excluded from any kind of national membership in a fuller sense. The Omani state has neglected developing its peripheries (Valeri 2017), while the appearance of border walls, checkpoints, and customs and immigration offices has made entry into the UAE more difficult—and consequently complicated borderland Omanis’ access to employment, education, and entertainment on the other side. Here, I argue that the recent materialisation of the border can help us understand why some borderland Omanis are now invoking historical and kinship ties to justify their pursuit of Emirati citizenship.