Why it's annoying:
If your entire response to someone's e-mail is "Ok," then you probably didn't need to respond at all. You should have at least taken the time to type a few extra letters so it reads "Thanks for letting me know." People are taking the time to read your message, so make it worth their while.
要是你的回复只有一个OK，那还不如不回复。你起码多写几个字，凑成一句“多谢告知”。别人花了时间来看你的回复，别让人白浪费时间。