"They are strange!"
"Jsou oni zvláštní?" is a word order of a question. If it was to be a statement, it would sound too poetic.
"Jsou zvláštní." is a neutral statement, if you want to emphasize that THEY are weird/strange, you put it in the initial position "Oni jsou zvláštní." or to emphasize the word "zvláštní", you say "Zvláštní jsou oni."
"Oni, ony and ona" all translate as "they". The difference between them is in grammatical gender. "Oni" is used for masculine animate nouns; "ony" for both masculine inanimate and feminine nouns; and "ona" for neuter nouns. On top of that "ona" can also mean "she" in singular, so watch out for the verb in Czech (if it's in singular or plural) to see which one is used.
When you have a group of people and it contains at least 1 male (ie. masculine animate), use "oni". When the group contains only women (ie. feminine) and children (ie. neuter), use "ony". The same applies also when constructing the past tense.