They both mean "they", but "oni" is the masculine personal gender, and "one" not masculine-personal.
Masculine personal refers almost exclusively to "a group of people including at least one male", and not masculine-personal to anything else, including a group of women-only, trees, dogs, boxes, etc.
Yes, it would make sense. Not only on its own, it's actually even a correct translation of this sentence.
Generally, forms of "tamten" (tamtego) translate to "that", and forms of "ten" (tego) translate to [this/that/the].
In terms of 'closeness', English uses this/that/that while Polish uses ten/ten/tamten. The middle forms overlap.
If a sentence uses 2 verbs consecutively and both those verbs require the object to have a different case to the other, what would we do?
Eg. The girl 'somethings' and 'somethings' this man.
If first verb 'something' required accusative, and second 'something' required genetive or instrumental or other, what declension would "this man" take?
For example, in certain duolibgo question, it asks to translate "They know and love that man".
If " to know" and "to love" required different cases, what case would "that man" take.
If both those verbs in this example above take the same case, then I mean an example where its two different cases.
I think I'd go with using the noun once and then a pronoun referring to that noun, because Polish hates repetitions. So for example "One znają tego mężczyznę (Accusative) i go (Genitive) potrzebują".
But the sole fact that I don't really know how to deal with it best seems to mean that it's just not a common problem ;)
I am really not sure. "potrzebują" should take "mężczyzny", after all...
Two years ago I asked about that sentence in the Facebook "Poprawna Polszczyzna" group, which deals with correct Polish. There weren't really many comments (so I guess people didn't know as well), the only suggestions were what I wrote above, or changing the whole sentence to passive voice.
In real life no one would probably bat an eye if you said your sentence, but both Duolingo and my actual job make me wonder about correctness of any grammatical construction ;)