What the specification means in practice
The OCI photograph borrows the American template: a 2 × 2 inch square, head 25 to 35 mm from the top of the hair to the bottom of the chin, eye height between 1 1/8 and 1 3/8 inches from the bottom edge. Then it diverges on the one detail everybody assumes. The background must be a plain light coloured background — not white. Applicants who reuse an Indian passport photo, which does require white, or a US passport photo, which requires white or off-white, get the size right and the background wrong.
What "plain light colored" means in practice
A light grey, light blue or cream background all satisfy it. The intent is the same as the UK’s: enough separation between the edge of the head and the background that the outline is unambiguous. Pure white removes that separation for anyone with pale hair or light clothing.
There must be no distracting shadows on the face or on the background. As with every specification here, standing a short distance away from the backdrop rather than against it is what prevents your own shadow from becoming part of the photo.
The eye-height requirement
OCI is the only specification on this site that publishes an explicit eye line as well as a head height. Your eyes must sit between 1 1/8 and 1 3/8 inches from the bottom of the photo — roughly 56% to 69% of the image height measured upward from the bottom edge.
That constraint interacts with the head height. A 35 mm head sitting low in the frame can satisfy the head rule and fail the eye rule at the same time, because the eyes end up below 56%. Getting both right means positioning the head vertically, not just sizing it.
Print quality
The photograph should be printed on thin photo paper or stock, with a clear, continuous-tone quality. Do not retouch, enhance or soften it. Blurred prints, ink marks and creases are named failure cases, and a photo printed on ordinary office paper will not have the tonal range required.
Framing and pose
Full face, front view, eyes open, head centred within the frame. The photo must present the full head from the top of the hair to the bottom of the chin — cropping the top of the hair is a rejection even when the head height measurement would otherwise pass.
The expression should look natural. Head coverings are permitted only for religious reasons, and the facial features from the bottom of the chin to the top of the forehead and both edges of the face must be clearly shown.
Before you submit
Check the background colour first — it is the error that costs the most time, because it is invisible to the applicant who is looking at the size. Then check that the head fills 25 to 35 mm of the 51 mm frame and that the eyes fall in the upper-middle band. Then check the top of the hair is inside the frame. Those three, in that order, account for most of what goes wrong.