is it possible to use S_ODT in Function Block?

monust

Also is it possible to use S_ODT in Function Block? If it is, how should it be defined to DB?

Yes. It is not to be defined in an interface.

There are two options:

1. The FB is not meant to have multiple instances - put an S_ODT in a code and hardcode a free S5 Timer number to it. 
2. The FB is meant to have multiple instances - create an input of type Timer, put a S_ODT in a code and assign the input as a Timer_No. Later on, when coding an instance of the FB a free S5 Timer number must be assigned to the input.

In general I discourage using S5 Timers in development as they are pain in a bottom part when it comes to code creation and reusability. In case functionality of an S5 timer is required, I'd advise to code an FB with such a functionality built on IEC timers.