Can anyone suggest me the programming of the mentioned subject? Its quite urgent
What have you tried ?
Which Arduino do you have ?
What OLED do you have ?
What programming experience do you have ?
What is to start the countdown? What will suspend it?
Do you want to count down in nanoseconds? Years? Centuries?
How often should the remaining amount be decremented?
What should be the initial value?
Your “requirements” are far too vague to be even considered a starting point.