Initialism of over-the-counter. / (slang) Initialism of off the charts.

(pathology) Initialism of Ornithine transcarbamylase deficiency.

(military) Initialism of Officers Training Corps (part of the British Army) / Initialism of Overseas Telecommunications Commission (defunct department of the Australian government)

(slang, Internet, text messaging) Initialism of on the cheek.