You should not buy tickets with OMIO. Buy them with DB directly.
When you buy a ticket you do not buy a ticket for a train, you buy a ticket for a route. If for some reason the train you selected is cancelled you still have a ticket. This ticket you now can just use on another train on the same route, and you do not even need to do anything for that. Just find the train, get on, and show your ticket to the conductor when he comes.
You should never have to pay extra because the railway did something....