In 1885, at age 27, he moved to Windsor, Colorado, either in search of better health, wrote Aven Nelson,[2] or to pursue a strong desire to study Rocky Mountain plants, inspired by one of his college professors, according to Roger Lawrence Williams.

"[2] His strong avocation, collecting native plants of the Rocky Mountain region, began in 1893.

[3][4] He consulted with professional botanists, especially Aven Nelson of the University of Wyoming, and Per Axel Rydberg.

Of these, 8,330 were of his own collections, and described by Roger Lawrence Williams as "testimony to leisure time arduously spent.

[6][7] According to IPNI,[8] his botanical author abbreviation, Osterh., is associated with 237 plant names or historical variations theron.