I guess I don't really know the capacity of the Oslo, but it sure looks tiny standing next to a Firelight. I can say 100% the two Firelight models are identical in castings, and pretty close in firebox size, both measuring 2.85 - 3.0 cu.ft., depending on what you count as "usable space".
IIRC, Ashford 30 is 2.85 cu.ft. Not huge when compared to the King, although classified as a "large stove" on BK's site. It does get burn times at least 2x the Jotul Firelight 12's, and probably 3x the Firelight 600's, in my short experience. Last night, I loaded 7 splits. 24 hours later, that load was only 70% consumed, and this big old house was holding 72F. Amazing, I couldn't come anywhere close to that with the Firelights.
I ended up running the stove on high for about 4 hours, just to burn down the remaining splits and get to a sensible reload point. Cat never went out of active range, and I just did a hot reload at 28 hours with plenty left in the box.
I think I'm going to have to get used to just doing a lot of hot reloads, on this stove. With the Jotuls, that wasn't much of an issue, as they were always down to coals at the end of an average work day.