In the ECOOP'97 conference, the author of the present paper investigated a conservative extension, called Ob+1<: , of the first-order Object Calculus Ob1<: of Abadi and Cardelli, supporting method extension in presence of object subsumption. In this paper, we extend that work with explicit variance annotations and selftypes. The resulting calculus, called Ob+s<: , is a proper extension of Ob+1<:. Moreover it is proved to be type sound.