Summary: A. Varchenko [Adv. Math. 97, No. 1, 110–144 (1993; Zbl 0777.52006)] introduced in 1993 a distance function on the chambers of a hyperplane arrangement that gave rise to a determinant whose entry in position \((C,D)\) is the distance between the chambers \(C\) and \(D\), and computed that determinant. In 2017, M. Aguiar and S. Mahajan [Topics in hyperplane arrangements. Providence, RI: American Mathematical Society (AMS) (2017; Zbl 1388.14146)] provided a generalization of that distance function, and computed the corresponding determinant. This article extends their distance function to the topes of an oriented matroid, and computes the determinant thus defined. Oriented matroids have the nice property to be abstractions of some mathematical structures including hyperplane and sphere arrangements, polytopes, directed graphs, and even chirality in molecular chemistry. Independently and with another method, W. Hochstättler and V. Welker [Math. Z. 293, No. 3–4, 1415–1430 (2019; Zbl 1428.52026)] also computed in 2019 the same determinant.