Abstract. Treating the standard model as an effective field with three weakly interacting neutrinos at energy below the center-of-mass energy of an experiment, neutrino mass arises from the dimension-5 Weinberg operator. The neutrino kinetic term can be further modified due to a dimension-6 operator, resulting in a nonunitary lepton mixing matrix U. On the one hand, this results in nonorthogonal neutrino flavor states, implying zero distance effect, i.e. there is a nonzero probability of a neutrino oscillates to a different flavor at zero distance. On the other hand, since the neutrino flavor state space is complete, in an experiment, the probabilities that a neutrino of a certain flavor being detected as all possible flavors, at any distance, should sum to unity. Using the naive Born rule with a nonunitary U, the total probability can be larger or smaller than unity. We will see how one can calculate the oscillation probabilities such that the total probability always remains unity.