There are a bunch of switchers in O scale, if you look around. Most are brass but there are others.
One of the best runners is the Atlas O SW8/9.
In Brass NJCB did all the Alco S series. The Carworks did a number of versions of the Baldwin VO 1000. Precision Scale did SW7 and SW1. I have seen a couple of different FM switchers. Also I have seen an Alco T switcher.
And of course the old Stand-bye, The All Nation NW2.
And there have been a number of GE 44 Tonners from various importers.