The point where the line touches the cue ball is aimed at the point on the front surface of the object ball. This method rules out errors in perspective and depth perception. It also works well for thin cuts along the rail. The OPO is not always the center of the pocket for several reasons. First, part of the pocket could be obstructed.
Snooker tables have less friendly pockets.
Players prefer big pocket opportunities rather than shot angles where pocket openings are obscured. Billiard tables vary in construction and even assembly. Table conditions will vary and don't anticipate playing on a new or newly recovered table.