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Not sure if “OPNS” in this context means OPNs (plural of Opn hearing aids) or OPN S aids. Respectfully, it doesn’t affect my opinion either way.
The Opn rechargeable aids did not use Lithium-Ion batteries; they used ZPower silver-zinc batteries. That’s a different power source that requires a different type of charger so of course they won’t charge in the Smart Charger.
The Opn S, on the other hand, is powered by a Lithium-Ion battery, and is charged by the Oticon induction Charger 1.0. The most current (2022) Instructions for Use for the Charger 1.0 state that it is compatible with Oticon Real, More, Zircon, Play PX, Opn S, Opn Play, Ruby, and CROS PX, all of which use lithium-ion batteries.
The Smart Charger came out later (summer-ish 2021). Regardless of what the current spec sheet or sales brochure states about compatibility, I’m not able to come up with any rationale that would explain why the Smart Charger is not capable of charging everything the Charger 1.0 can charge. (Interestingly, the Charger 1.0 Technical Data Sheet lists compatibility only with the Real, More, Zircon & Play X aids, which shows the documentation doesn’t always match compatibility).
In any event, this is a bit tangential to the original issue of whether the Smart Charger and Philips/Bernafon Charger Plus are interchangeable. It’s my opinion that as long as so long as your Oticon/Philips/Bernafon Lithium-Ion aid will fit into the receptacle in the charger with a reasonably close fit you should be able to use any of the three.
P.S. I’m posting my thoughts & conclusions about what I’m comfortable doing, but respect that others might not wish to do the same.