AC is tangent to circle O at point C. What is the measure of O?

AC is tangent at point C, which makes a right angle with line OC.
That means that angle C = 90 degrees.
The 3 angles of a triangle need to equal 180.
Subtract the two known angles from 180 to find angle O:
O = 180 - 90 - 23 = 67 degrees.