Can "hold on" and "hold off" be used interchangeably?

These are two different words but I've often seen them used interchangeably in various situations. For example:

1. I cannot hold off the fort any longer than this.
2. I will hold on to my faiths and beliefs whatever happens.
3. Let's hold on for some time, this draft isn't needed this very moment.

Interesting question.

Holding something off is generally keeping it away. Holding off or holding back the enemy's attack. Holding on to is keeping it close by, preserving it carefully. Holding on to your closely held beliefs. There is also holding out as in your number 3. They should be holding out for more time they need. They are holding on to the slim chance for getting more time. I hope they get it.