I am at the beginning stages of collecting I scale trains for my future layout (though I have some already). But I am doodling around with track plans and such now. Being new to O gauge (three rail), I wasn't aware that you could run conventionally controlled engines with a wireless system which I will definitely want on my layout as it might not be possible to operate it all from one area.
Previously I inquired as to why the mix of command and conventional-only engines in your HO layout. My curiosity was whether, for example, the conventional-only engines had personal meaning such as your first engine and you wanted to keep it that way, or if you just like the hands-on throttle of conventional control, or it was not readily upgradable to DCC, or whatever.
Likewise, perhaps you want to leave your conventional-only O engines as-is for similar reason(s).
IMHO, since the Original Poster is already familiar with using the non-proprietary, world standard, DCC system -- he might consider just CONTINUING with DCC.
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... Is there a way to run conventional control engines using a command system that has a walk-around throttle so that you aren't permanently tethered to one area (loss of some of the fancy sound effects is okay)?.
James brings up an interesting idea. You could install low-cost DCC decoders into the conventional-only engines. Then you would have wireless control of your "conventional" engines!
As you commented, if you are willing to give up some of the proverbial "bells and whistles" to achieve walk-around control, there are very reasonably priced DCC decoders with suitable "Amps" of power to drive O engines. To be sure, it's not as simple as removing the NMRA jumper and plugging in to the decoder socket; I am not aware of any O-gauge engines that have such a simple un-plug, plug, and play conversion from conventional-to-command. So there would be some DIY wiring, soldering, etc. which can be a show-stopper for some. But O-gauge engines have so much more internal space to fit the electronics. So while the scale-ratio is 87:48 or 1.8 times...that's for each dimension (LxWxH). The internal volume of an O-gauge chassis about 6 times greater than an HO chassis!
You might need a DCC power booster for O track power. And perhaps an unused benefit, but this would allow you to run both your HO and O layouts - even at the same time - using a single wireless controller you already have and know how to use.
I'm repeating myself, but I think it's worth pondering the mix-and-match of engines and decoders in the non-proprietary DCC world vs. the proprietary Lionel/MTH systems where you buy both engine and decoder from the same manufacturer. There are plusses and minuses each way.