Object-oriented code in F# (2012)

As should be apparent, you should generally try to prefer functional-style code over object-oriented code in F#, but in some situations, you may need all the features of a fully fledged OO language – classes, inheritance, virtual methods, etc. Just as in C#, F# can extend existing classes with extension methods. F# classes can have events, and the events can be triggered and responded to.

Source: fsharpforfunandprofit.com