Skinny . . . you've got some good advice here from a number of folks with actual experiences with the Oslo . . . and some lousy advice from some folks who don't heat with Oslos . . . and some good and decent advice from folks who don't heat with the Oslo, but have had similar experineces . . . you'll have to weed out the good from the bad.
That said, as mentioned all maples are not created equal . . . on the top end is sugar maple . . . on the low end is silver maple. I think you'll find red maple somewhere in the middle when it comes to BTUs and the maple family.
Inconsequential advice . . . it's a new stove, right? I highly doubt this is a problem with gaskets and seals. I think it's more of an issue with the operation and limitations of the fuel, type of stove (i.e. secondary burner) and size of the firebox . . . but don't worry . . . we'll get you through this and have you waking up to coals in short order.
Tradergordo had some fine advice on the definition of an overnight burn . . . for a firebox the size of the Oslo it is unrealistic to expect to go to bed and wake up 8 hours later to a firebox still full of flames no matter how much wood you stuff into it, how well seasoned the wood is, what species the wood is, etc. If you can go to bed and wake up 8 hours later to find a warm stove and rake up some coals to restart the fire I would say this is about as good as it can get . . . that's the bad news.
The good news is that your stove will keep the house heated moderately well . . . providing it is insulated well . . . and you will and can wake up to coals the next morning with a few easy tips . . . most of which have already been noted here.
Last year I thought I had great, well-seasoned wood. I was wrong. It was better than many newbies who were burning last year . . . but now that I'm burning truly seasoned wood this year I have seen the difference as the fires are hotter, faster and the secondaries last longer.
Last year I thought I was running my stove well. I was wrong. I was cleaning out the ash in the firebox too often . . . now I realize that keeping an inch or so of ash in the firebox is the key to having less heat on the hearth, more heat in the room and to waking up to coals in the morning.
Webby offered some great advice . . . as did Smokey Foot . . . they pretty much do what I do . . . load up the stove a little before you go to bed, after the fire has been established for awhile. Get the stove temp up to 500-550 (in my case) and then begin to slowly throttle back the air. Last year I could only go down to a 1/4 closed . . . this year with much drier wood I have been able to close the air all the way which has made a big difference. Now I know you're tired at this point and want to go to bed, but I would encourage you to stay up just a little longer because watching the secondary action at this point is incredible . . . eventually when you get tired go to bed . . . and wake up 6-8 hours later to find some coals . . . you may have to stir the ashes a bit . . . but they should be there.
For me the three keys to achieveing my goal of keeping the stove warm and getting coals the next morning are:
a) Ash . . . you've got to have an inch or two in the firebox . . . don't worry about covering up the grates . . . the ash will be there naturally once you have a few fires in the firebox.
b) Wood . . . you've got to have good, well-seasoned hardwood . . . although Smokey Foot is getting by with pine (no doubt he is bald and has hairy palms from burning all that pine
. . . bigger is generally better . . . at least have a good sized split or round in the back and medium stuff in the middle. More is also good . . . the more wood in the firebox = more of a chance you'll have coals in the a.m. Try switching to some better wood . . . hemlock and birch are not great for overnight burns. You mentioned elm . . . if it's seasoned try that or some ash if you have it.
c) Air . . . the key (contrary to what some folks have said here) is to have well seasoned wood and use the air control to gain both BTUs and time . . . getting the firebox up to temp and then throttling back the air will result in maximum BTUs coming from the stove . . . and the secondary action will last longer . . . trust me . . . I've seen this. Using green wood may result in longer burns, but it can suffocate the fire and create more creosote.
The key really is utilizing the ash, getting well seasoned wood so you can get the seconedary action and then using the air to give you the time and BTUs where you want it -- your home.
I wish I could have had you stay overnight with me . . . last night I threw some "junk wood" into the firebox around 9 or 9:30 when I went to bed . . . mostly some odd ball pieces and chunks of elm, maple and ash . . . didn't load the stove to the gills or anything . . . woke up this morning around 4:30 . . . no glowing coals . . . but when I stirred the ash there were enough embers to get some newspaper and kindling going without using a match . . . within 10 minutes I had the stove back up to 450 or so (of course it was only down to 150 or so when I started.) As I said . . . this was done without loading the stove up with the premium wood or using large splits or rounds . . . just chunks . . . the key however was I had plenty of ash in the firebox from the fire my wife had going all day long.
