|OA|=|AB| = 2
|OB|=1
By cosine- rule
AB^2 = |OA|^2+|OB|^2-2|OA||OB|cos∠AOB
4=1+4-4cos∠AOB
cos∠AOB = 1/4
OA.OB = |OA||OB|cos∠AOB
= 2(1)(1/4) = 2