Exactly where my mind was going, Hogz. I’d probably avoid the Jotul Oslo, which is just about the worst stove you could buy, if you’re putting it into a masonry fireplace. The masonry will suck up almost all the heat that stove produces, I know because I did this for several years with my small fleet of Jotuls.
If that’s a masonry fireplace, and if it is on an exterior wall, then you want a stove that’s going to radiate as little as possible off the sides and back. This completely rules out any Jotul cast stove (except the cast over steel designs), and it also rules out soapstone. In fact, it actually favors in an insert, or a very convective stove (eg. BK 30 or PE Alderfea). I know Jotul has some cast over steel designs (F45/50/55?), but I do not know if they’re completely clad on sides and rear. Can someone confirm?
If that’s a steel firebox, then the issue is much reduced, as the steel has less capacity for soaking up radiated heat than stone. Also, if the fireplace is in the center of the house, rather than on an exterior wall, the problem is reduced.