All of my experience with boilers is with gas fired commercial boilers. Here in Winnipeg, everyone uses gas fired heating equipment, whether it be boilers or furnaces. The odd place, usually commercial buildings like strip malls use electric heat.

I'm surprised to learn that oil fired boilers can be converted into gas fired units, and vice versa, but it's clear that the boiler would have to be designed for either fuel right from the start, and I've never seen that. Perhaps it's more common in more expensive boilers like Viesmann. But, if you go onto a more common boiler manufacturer's web site, like Weil McLain, they classify their boilers as gas burning and oil burning, and so far as I've seen, they don't have a catagory for either/or.

Anyhow, now the OP is aware of the possibility and can check to see if it's a feasible option in his/her case.