In this paper, we generalize the concepts of (intra-, left, right, completely) π-regular semigroups without order to po-semigroups and discuss characterizations and relationships concerning them. Moreover, we introduce the concepts of nil-extensions of po-semigroups first, then consider properties and characterizations of po-semigroups which are nil-extensions of (left, rightt-) simple (π-regular) po-semigroups and complete semilattices of this class of po-semigroups.