1) metaphor 2) zeugma: "Splash" controls many words (none of which are really grammatically correct): "water splash" "splash over the top" It also asks to be misinterpreted as a verb: "water splashED over the top" 3) irony 4) Could be irony, but it isn't really all that ironic because everybody is scared of fire. 5) oxymoron

1) isn't oxymoron because there's no apparent contradiction anywhere. I think it's hyperbole because it's exaggerated. It's NOT completely impossible that the speaker might change their mind and start dieting. 2) "Splash" is indeed an onomatopoetic word. There are many omomatopoetic words referring to water: splash, gush, spurt, tinkle, bubble, simmer, drip... even "wash," now that I think of it. 3) This is definitely irony and definitely not oxymoron. "Irony" is complicated and has more than one meaning. The Wikipedia article on "irony" is very good. This is an example of "cosmic irony," where fate or the universe does the opposite of what you'd expect. You are right that #4 is similar. 5) This is your only example that is an oxymoron. The speaker, at a loss for a direct word that is strong enough to express how delicious the cake is, intentionally uses a word that means the exact opposite. It's clever because eating too much cake can make you feel sick, so it suggests that the cake is so good you will overeat. (In #5, "digustingly" should be "disgustingly," and in #2, "splash" should be "splashed." "Irony" is tricky because it means many different things. "Oxymoron" is tricky because some years ago, someone invented a form of humor that consists of calling things oxymorons. They are jokes. And they aren't real oxymorons, because they aren't intentionally opposite. For example, consider the joke "'Military intelligence' is an oxymoron." "Military intelligence" means acquiring information about the enemy's battle plans. Calling the phrase "an oxymoron" is an indirect way of saying "The military is not intelligent" or "Generals are stupid." Notice this also involves using a second meaning of the word "intelligence."

(replying to your reply) The main meanings of "irony" are a) "a strange and self-contradictory twist of fate," and b) "saying the opposite of what you mean, knowing that your audience will understand your real meaning." Examples of the first meaning: "Dr. Guillotine, inventor of the guillotine, was executed by the guillotine." (That's not actually true, but it's a good example of irony). "Ironically, one of the best-loved and most productive research buildings at MIT was Building 20. It was built quickly during World War II and was intended to be a temporary structure. It lasted 55 years.) During a scare about the disease anthrax, the leader of a rock band named Anthrax said he had decided to take an antibiotic because "I have vowed that I will not die an ironic death." (It would be ironic for the leader of the band Anthrax to die of anthrax). An example of the second meaning: A blog by crypto skeptic Molly White is entitled "Web3 is Going Just Great ...and is definitely not an enormous grift that's pouring lighter fluid on our already smoldering planet." It is a report on crypto scams, disasters, and fraud. She means, of course, that it is NOT going well, and IS an enormous "grift" (swindle, cheat, fraud). Notice that this is an example of both sarcasm and irony. It is a remark expressing contempt, anger, and mockery; that makes it sarcasm. It does it by saying the opposite of what it means; that makes it irony.