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Hi,
Can I use "Ok/Okay" in formal way of writing? If not, what is the appropriate word to be used in place of Ok/Okay?

Regards,
Sri
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They are not formal. What you use instead depends on the sentence structure:

This equipment is satisfactory/operational.
Yes, you can return to the office early if you have finished.
I am fine, thank you.
etc.
Comments  
Mr.Micawber,
Thanks for your suggestion.
Regards,
Sri