The Santali spoken language is not an Indo-European language, while the majority spoken languages in northern India are Indo-Europeean. (Santali is an Austro-Asiatic language, and hence more closely related to Vietnamese than to Sanskrit.) The Indic writing systems designed for Indo-European languages were thus deemed to be unsuited to Santali — the consonants weren’t right, there wasn’t a sign for the glottal stop, and there weren’t enough vowels. Similarly, the Latin script didn’t have a glottal stop sign or a way of indicating vowel length.
Me, I think they could have gotten around those issues by adding characters or diacritics, or doubling vowels, but Pandit Raghunath Murmu decided to develop a script in 1925. He introduced in a book titled “Ol Cemet” (“Language Learning”), and so the script is sometimes called “Ol Cemet”.
Ol Chiki is an alphabetic script, and one of the very few alphabets in the world that does not descend directly from Greek. However, with English being widespread in India, clearly Murmu knew how alphabets work.
Murmu designed the glyphs to look like things that incorporated that glyph’s sound, and used the name of the object to be the name of the character. For example, the word for “mushroom” is “ud”; there is a character named “ud” that looks like a mushroom, and which represents “d” (or “t” when unvoiced). The glyph for the character named “aag” is designed to look like the shape of the mouth when throwing up, and it represents is a choking sound: “g” (or “k” when unvoiced).
Like the abugidas in the area (northeast India), Ol Chiki uses several diacritics on its vowels to change the length, nasalization, and vocalization (whether it is voiced or unvoiced).