an oxymoron is a contradictory phrase like 'deafening silence' or 'cruel kindness'
antithesis is the complete opposite of something, it can be a word or character, setting etc like heaven is the antithesis of hell or god is the antithesis of the devil (lol not religious but they're the only examples I can think of right now haha)
juxtaposition is when two things are contrasted- it can be words, settings, chapters etc for example "schoolmasters can puzzle their brains with nonsense and learning' - nonsense and learning are juxtaposed to highlight the author's stance on education
Hope that helps