Chlorine has the following % abundance and mass:
Cl-35 = 75.76 %, 34.9688 amu
Cl-37 = 24.24 %, 36.9659 amu
Oxygen has the following % abundance and mass:
O-16 = 99.757 %, 15.9949 amu
O-17 = 0.038 %, 16.9991 amu
O-18 = 0.205 %, 17.9991 amu
Based in the % abundance O-16 is the most abundant oxygen isotope and Cl-35 is the most abundant chlorine isotope
So the most abundant molecule of Cl2O and their respective mass are as follows:
1) Cl-35, Cl-35, O-16
Mass = 34.9688 amu + 34.9688 amu + 15.9949 amu = 85.9325 amu
2) Cl-35, Cl-37, O-16
Mass = 34.9688 amu + 36.9659 amu + 15.9949 amu = 87.9296 amu
3) Cl-37, Cl-37, O-16
Mass = 36.9659 amu + 36.9659 amu + 15.9949 amu = 89.9267 amu