Can Arm-R-Seal be applied over the top of a single coat of oil/wax?
As it stands, no, but with surface prep yes.
There's a rule in finishing, nothing goes over wax but wax. Wax is a resist, nothing will stick to it properly but more wax.
Now obviously you've used an oil/wax blend which isn't just wax, but there's still wax in there and it will affect adhesion of a varnish topcoat.
The ideal fix here would be to skim off the surface of the side tables, just removing a few thou should be enough to get below the treated wood fibres (penetration of all finishes is surprisingly shallow in nearly all woods, except on end grain). This can be done by sanding, scraping or planing, or a combination, whichever process(es) you prefer.
It's probably a good idea to then clean the freshly surfaced wood down with solvent to remove any last traces of the existing finish that may remain, unless you go deep enough that you're sure you've exposed pristine wood. If you want to do the solvent thing start with a few wipes of mineral spirits then finish up with something stronger like acetone or lacquer thinner.
You might possibly get away with just thoroughly cleaning the surface with the solvents, but I wouldn't be inclined to rely on this alone myself and it would require copious amounts of solvent.
prevent water stains
Another way of looking at this: do your end tables actually need protection from water?
If they're to be used regularly for holding drinks then a polyurethane varnish is a great choice as a topcoat, but if they aren't then the oil/wax blend you've used already is a perfectly acceptable finish for them.
Arm-R-Seal in satin is a really popular choice
Personally I would recommend you get gloss varnish, thin it to wiping consistency and then wipe it on. You'll naturally get a satin finish this way, with a gloss finish available for any future project without a subsequent purchase being necessary.
Just wanted to add a couple of bits on this in case it's of use or interest.
Thus far, I've worked on cutting boards and finished with a combination of walnut oil/beeswax for a food safe finish.
First, note that most cutting boards don't actually need to be finished in any way to withstand use and regular washing. Finishing is nearly entirely done for looks, not because boards actually need it to prevent them falling apart, cracking, etc.
Now about "food-safe" finishing, there's masses of humming and hawing about this online but virtually all wood finishes are broadly considered food-safe by regulatory agencies worldwide once fully cured. This includes varnishes and lacquers.
While neither of them seem at first glance to be suitable finishes for a cutting board (because you don't want an actual film on the surface which can be cut and start to chip, possibly ending up in the food*) if thinned heavily they will soak into the wood.
*Even if you did build a film and it did chip and some got ingested this is still no big deal — film finishes are passed for use on children's toys, that's how safe they are once fully hardened.