「占」 was originally a combination of mouth 「口」, representing words/diction, and either pyromantic divination cracking 「卜」 (Series B) or a bone with pyromantic divination cracks 「𡆥・⿴囗卜」 (Series A). The whole character represents pyromantic divination of prophetic words.
The word for divination cracks was originally written as 「𡆥・⿴囗卜」
made up of divination cracking 「卜」 in a shoulder bone or scapula 「肩」 . The entire character is a simplified depiction of an oracle bone; see Why's flesh (⺼ ) the semantic component for 肩, 脖? for more details.
This character no longer survives in the modern writing system, being replaced by the unrelated glyph 「兆」, which is used a phonetic loan for the original word for divination cracks.
For an idea of how the divination process works, see e.g. 《殷虛文字丙編》８、９:
丙8 is an oracle bone made from turtle plastron, and 丙9 is the reverse side of 丙8. The holes drilled in 丙9 lead to cracks appearing in 丙8 when heated; the small numbers 一、二、三、亖（四）、𠄡（五） show the number of "divination rounds" that were performed on this bone, and 二告 on the bottom-left hand corner of 丙8 (between 亖 and 五) was a re-attempt at a divination round which the oracles or diviners felt were failed attempts at communicating with the spirits.
Divination on the day of bǐngchén, Oracle Què asked: "Will we receive millet harvests?"
Divination on the day of bǐngchén, Oracle Què asked: "Will we not receive millet harvests?"
The King divined, prophesising: "It is auspicious, and we will receive the harvests."
- The prophecy and its opposing result are given as dictions inscribed on to the oracle bone;
- Cracks will start appearing after intense heat is applied to the bone, from the drilled holes towards the centre;
- A diviner (in this case, the King) will interpret the crack patterns surrounding the inscribed prophecies, and determine whether the auspicious or inauspicious prophecy will come to fruition.