In the bottom ABCD, EF is perpendicular to OB, plane geometry, self-proof. BB 1 is perpendicular to the bottom ABCD, and EF belongs to the bottom, so BB 1 is perpendicular to EF, and of course EF is also perpendicular to BB 1.

To sum up, EF vertical OB, EF vertical BB 1, OB and BB 1 intersect at b, so we can know EF vertical plane OBB 1.