Re: So I have some Osnaburg...
O & I have had many adventures. The actual washing instructions off the bolt indicate cold wash on gentle. A warm wash is sufficient to shrink it, and if it wasn't the overnight soak in hot water and oxy would be. Once it's shrunk that's it- it's shrunk. It won't stretch back out or shrink anymore (at least not significantly, a few centimeters loss for wash and stretch after wear are normal variances).
Hot, or warm, just be sure that you preshrink it.