In "O SCale" there are a lot of companies making different things. If you look at N, most of the "million" make cars, or locos, or track, or electricals, but few making everything. The full line business that tries to pull you into using only one manufacturers products such as locos, track, controls, cars etc is more of a feature of 3 rail trains
In O scale we have Stevenson Preservation, Sunset 3rd Rail (and GGD), Mullet River, Old Huff and Puff, Labelle, BTS Car Shops, Key, OMI, Atlas, Weaver, NWSL, Funero & Camerlengo, Precision Scale, Southern Car & Foundry, San Juan Car Co, Kadee, Athearn, St Charles Model Works, The Carworks and Old Pullman that I can think of right off. Since the "scale" manufacturers follow NMRA standards and recommendations for the most part, everything is compatible. And most of these smaller outfits, do not supply whole lines, but rely on other companies to provide other components. For Example Sunset makes really nice locos and some cars but no track or control systems. NCE, Digitrax, Lenz, and CVP make great DCC control systems for O scale. Old Pullman, Micro Engineering, and Atlas make track as do some others. Weaver tends to make more cars than locos but does do both but no track or controls either. Many of the others make kits for various things on the Model RR.
So I think we in O Scale have quite a few manufacturers, but as you say, not as many as HO or maybe even N.
