Yea, they opened their mouth wide against me - See the notes at Psalms 22:13.
And said, Aha, aha! - See Psalms 40:15; Psalms 70:3. The language is that which we use when we “detect” another in doing wrong - in doing what he meant to conceal.
Our eye hath seen it - We are not dependent on the reports of others. We have seen it with our own eyes. We have found you out. We cannot be mistaken in regard to it. The reference is to some supposed “detection” of misconduct on the part of David, and the joy and triumph of such a supposed detection.