We know that the sum of any two sides of a triangle is greater than the third side.
In AOB:
OA + OB > AB...........(1)
In BOC:
OB + OC > BC........................... (2)
In AOC:
OA + OC > CA.............................(3)
Adding (1), (2) and (3):
OA + OB + OB + OC + OA + OC > AB + BC + CA
2( OA + OB + OC) > AB +BC + CA
Hence, proved.