It is easier to oxidise water than Cl- ions according to the standard reduction potential, then why Cl2 is liberated at anode instead of O2??

At anode oxidation occur and Cl(-) is selectively discharged because of its high concentration and the negative ions, chloride and hydroxide, get attracted towards the positively charged anode. It is actually easier to liberate hydroxide ions (to give oxygen) than chloride ions (to give chlorine), but there are far, far more chloride ions arriving at the anode than hydroxide ions.
The major reaction at the anode is therefore:
2Cl(-) – 2e(-) ---------> Cl2
Two chloride ions each give up an electron to the anode, and the atoms produced combine to give chlorine gas.
The chlorine is, however, contaminated with small amounts of oxygen because of a reaction involving hydroxide ions giving up electrons as well.
4OH(-) – 4e(-) ------> 2H2O + O2