We have chosen an Ising ASYNNNI (ASYmmetric Next Nearest Neighbor Interaction)   model under a grand canonical regime to investigate structural phase transition from a high symmetric tetragonal (Tet) to a low symmetric orthorhombic in YBa₂Cu₃O_6+x , 0c system. Ordering process for absorbed oxygens from an external gas bath into the basal plane of the layered system is studied by Monte Carlo Glauber dynamics which focuses on the effect of chemical potential in the bath on the oxygen diffusion inside the basal plain, as well. It is predicted short range super-lattice phases called orthorhombic II and III (OII, OIII) and also a long range orthorhombic I (OI) phase, corresponding to the oxygen stoichiometry, x, and temperature. It is consistent with literatures and confirmed by our ND studies (ILL, France) too.