Hello Alex,
The reason the description is so vague is because its possible the ram installed from factory could be several different kinds.
In order to add a single stick to the existing, you would have to use a tool like CPUz and read the timings of that stick.
Then match the old with the new exactly. Otherwise it might have problems.
Or you can just purchase all new ram if you wish and bypass any compatibly issues. It uses DDR4-2133Mhz up to 32GB total, but you can simply install a matched pair of DDR4-2400 which will be the same price basically.
Crucial is what most of us recommend only because they have had such a good track record of warranty if it didn't work.
This is what they recommend.... https://www.crucial.com/usa/en/compatible-upgrade-for/HP-Compaq/omen-870-224#MEMORYFilters
You can always find the model ram you want there, then look for it on Amazon... 😉
