∴ ∠ACB = 40°
We know that angle subtended by arc of a circle at center is double the angle at remaining part.
∴ ∠AOB = 2∠ACB
= 2 × 40°
= 80°
⇒ ∠AOB = 80°
∵ OA = OB = radius of circle
∴ In ∆AOB
∠OAB + ∠OBA + ∠AOB = 180°
∵ Angles opposite to equal sides are equal.
∴ ∠OAB = ∠OBA
⇒ ∠OAB + ∠OAB + 80° = 180°
⇒ 2∠OAB = 180° – 80°
⇒ ∠OAB = \(\frac { { 100 }^{ \circ } }{ 2 } \)
⇒ ∠OAB = 50°