I looked up the manual for the Oslo and it's the same as the castine. Optimal is 400-600. Any part of the stove glows red is bad and never open the ash door during a fire (even though I do for short bursts to get things going occasionally) otherwise you run the risk of cracking the stove.
I think a number of Oslo owners have done this when first starting out . . . and they either read more here and see pics of damaged stoves and immediately change their wayward ways and eventually discover that leaving the side door ajar may take a bit longer, but works almost as well (especially when paired with nice, seasoned wood and a Super Cedar) . . . or they disregard the advice or never stumble on to Hearth.com to learn better and eventually their stove is damaged and they're mad at Jotul for their "cheaply made" stoves.