Range
Regarding the difficulty in separating O. nigricornis from O. forbesi based on photographs: A gal who has done extensive studying of tree crickets (including song analysis, mating trials, and DNA sequencing) once wrote to me: In the eastern United States, I have found O. nigricornis, but not O. forbesi (in NJ and three sites in NY). In Ohio, I have found both O. forbesi and O. nigricornis in the same field, although the sites where I have found this have been in eastern Ohio (Akron and Canton). In central Ohio (Columbus area) and west from there, I have found O. forbesi but not O. nigricornis (OH, IL, WI, MI, IN, SD).
It's probably wise to use this broad range for now in separating the two species. Photos taken in or near Ohio should probably go in the nigricornis vs forbesi taxon page.
Remarks
A great source for 'everything you ever wanted to know about tree crickets' is an article written in May 1915 by Bentley B. Fulton in a Technical Bulletin for the New York Agricultural Experiment Station.
The Tree Crickets of New York: Life History and Bionomics
