Why are the molecules of hydrogen chloride gas (hcl) more tightly bound to each other than molecules of oxygen gas (o2)?

Answer : If we consider the molecule of oxygen gas which in diatomic state, is bonded to other atom which is of same element this is called as homonuclear.

While in HCl there is a heteronuclear bonding observed because there are two different elements getting involved in the bond formation, also  it creates a electrovalent species in itself and makes it more polar. They are creating a dipole moment by separating different charges in the molecule which cause it to get tightly bonded with each other.

The chemical bubbles