Formal:
* O.K. - Often used in formal writing, especially in abbreviations and acronyms.
Informal:
* Ok - A common abbreviation, especially in informal contexts like texting or online chat.
* K - Very informal abbreviation, often used in texting.
* Alright - A more common alternative to "okay" in informal speech and writing.
Historical:
* O.K. - Originated as an abbreviation for "oll korrect" (all correct).
* All right - The original two-word phrase that "okay" is derived from.
Other:
* Kay - A casual pronunciation that sometimes gets written down.
* A-ok - Used to emphasize that something is perfectly fine.
It's important to note that the formality of these spellings varies. "O.K." is the most formal, while "K" is the most informal. Choose the spelling that best suits your context and audience.