Produce AO to meet the line CB at E. Then
CA + CE > OA + OE (shortest distance between 2 points is a straight line)
BE + OE > OB
Add these inequalities,
CA + CE + BE + OE > OA + OE + OB
CA + CB + OE > OA + OE + OB (CE + BE equal CB)
CA + CB > OA + OB (subtract OE from both sides)