I'm not sure what that building was. It could have been part general store. I'd say that that is right where the modern eating facilty is now. The tracks are to the right of the fences. And more stock pens were (and are) to the right of the tracks.
There was never a roundhouse or engine house at Osier. Just the covered turntable that was connected to the station. It had sloping sides and no windows and was very similar to the covered turntable at Cumbres. There was a bunkhouse too. That is where we park our cars now.
By the way, this picture is online at the Denver Public Library's Western History Collection and was taken by a Louis Hollard.
Richard Dorman's books have quite a number of old Osier photos.
And if I could make out the writing, I may be able to identify the building in the old ROW maps. I recently noticed that the old editions of Doris Osterwald's Ticket To Toltec had some of these maps. They were taken out in the later editions.