After years of resistance, I helped a friend move, and spotted a guitar he'll never play (he's an inveterate buyer of toys he'll never use). He turned me down, and 6 months later when I was helping a mutual friend move, he changed his mind. So now I have a $500 guitar for 200 bucks. And I need to learn to play it.

To start with, I've turned to the songbooks I already own which have chord diagrams at hand. A Dubliners songbook, the Stan Rogers songbook, and the Songs and Dances of Ireland and Scotland (The yellow books, if you know them)

Some of the diagrams have Os above some but not all of the open strings. Others have the same fingerings and the same name, but have no Os. Others have Xes on some open strings. None have both Xes and Os.

So how should I interpret these? Hit the Os but avoid the Xes? Ignore both and strum all 6? Wait until I've got the fingering nailed before I work on skip-string strumming patterns?

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And now there was no doubt that the trees were really moving - moving in and out through one another as if in a complicated country dance. ('And I suppose,' thought Lucy, 'when trees dance, it must be a very, very country dance indeed.')

C.S. Lewis

Yes. But avoiding the Xes at all cost is not always necessary - they may disappear in the general noise. Depends on the chord. If it doesn't sound bad then don't worry. It'll become easier with time in any case. Is it acoustic or electric? For electric guitars you mute the string(s) instead of not playing them - with the heel of your right hand, or a finger on your left, or some other way - players have their own ways (on electric guitar it's much more important to mute those strings. On acoustic you can mostly get away with not doing it.)

Boy, some friend you are. Was he stuck there all that time because you wouldn't help until now?

https://www.justinguitar.com/en/BC-108-TABandBoxes.php

Watch the video, or scroll down to the diagram: