What does "Ok then" mean? Can one use this in his or her conversation? Will this offends the hearer? I just used this while mailing my boss. Will that be ok?
In broad terms, it mans that you agree because of some reason that has been given.

It's not inherently offensive, but only say it if you have a friendly and very casual relationship with your boss.

emailing, not mailing.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Dear Clive, thank you very much for your kind reply and for your correction (re email). In my email I had written "Ok then, enjoy your weekend and convey my regards to your family.". So you say that I don't have to feel bad and apologize? I know him for past 5 months and our relationship is not very casual but fine.
You don't need to worry about it

CliveEmotion: smile
Dear Clive, that's a relief. Thanks again. Have a wonderful weekend Emotion: happy.
Teachers: We supply a list of EFL job vacancies

you see, it depends on how you use it. if you use it a conversation and youuu look annyoed than that can be taken offensivly. if youu are eamiling someone than like [a boss} he wont take it o