operator abstraction is a language construct relevant to object oriented programming languages and to ML2000, the successor to Standard ML. In this paper, we introduce Fω ≤, a variant of Fω <: with this feature and with Cardelli and Wegner’s kernel Fun rule for quantifiers. We define a typed operational semantics with subtyping and prove that it is equivalent with Fω ≤, using a Kripke model to prove soundness. The typed operational semantics provides a powerful tool to establish the metatheoretic properties of Fω ≤, such as Church–Rosser, subject reduction, the admissibility of structural rules, and the equivalence with the algorithmic presentation of the system.