https://www.inaturalist.org/flags/520318

Stotler, R.E. and Crandall-Stotler, B. 2017. A snyopsis of the liverwort flora of North America north of Mexico. Annals of the Missouri Botanical Garden 102(4): 574-709.

pg. 661 Stotler and Crandall-Stotler explain this synonymy based on two molecular studies Vilnet et al. (2012) and Aranda et al. (2014), as well as the interpretation that O. prostratum and O. sphagni are different morphological and ecological expressions of one species (Schuster 1974).