A hole too close for the bolt head: a school-boy error, which we all make from time to time !
Use a side cutting tool bit* and 'slot' the holes diagonally out towards the corners, keeping the width of the slot the same as the diameter you require, but thus allowing room for the bolt head and a washer.
(@MG: The picture of the bolt hole might look oval, but that is almost certainly owing to the distortion caused by the wide angle lens of the camera and the angle the shot was taken from.)
*I don't know the correct term.
Edited By John Doe 2 on 13/10/2022 10:14:43