Formal:
* Acceptable: Suitable or satisfactory
* Satisfactory: Meeting requirements or standards
* Alright: Acceptable, though not particularly good or exciting
* Adequate: Sufficient for a particular purpose
* Passable: Of a quality that is just good enough
* All right: A more formal way of saying okay
Informal:
* Fine: Good or satisfactory
* Good: Of a high quality or standard
* Great: Excellent or very good
* Cool: Good, impressive, or stylish
* Sweet: Excellent, enjoyable, or pleasing
* Sure: Used to express agreement or acceptance
* Yep: An informal way of saying "yes"
* Alright: Acceptable, though not particularly good or exciting
Neutral:
* So-so: Neither good nor bad
* Fair: Of an acceptable standard, but not particularly good or bad
* Mediocre: Of only average quality
* Decent: Of a reasonable quality, but not outstanding
The best synonym for "okay" will depend on the specific situation and the desired level of formality.