Odorant receptor (OR) expression is monogenic in most species. In mouse, an OR feedback loop generate the monogenic expression. In adult Drosophila OR expression is stereotype and suggested to be predetermined, not requiring feedback regulation. Here, we will present an OR feedback mechanism also in Drosophila that early in life refines OR expression. We will show how dLsd1 and Su(var)3-9 balance heterochromatin formation to direct OR expression similar to mouse. We will show that the expressed OR induces dLsd1 and Su(var)3-9 expression, linking OR level and possibly function to OR expression. OR expression refinement shows a restricted duration, suggesting that a gene regulatory critical period brings olfactory sensory neuron differentiation to an end. Consistent with a change in differentiation, stress during the critical period represses dLsd1 and Su(var)3-9 expression and makes the early permissive OR expression permanent. We will further discuss what such a mechanism can do for OR evolution and how the induced permissive gene regulatory state makes OR expression resilient to stress. We will also discuss how the Drosophila critical period fit with mouse OSN differentiation and that the early steps in monogenic OR expression regulation is not so different across phyla.