1) Not all plants give off oxygen.
2) Refer to 1).
3) Some plants can kill you if you ingest them.
4) And so is my starfish.
5) Any more [ad hominem inserted here] that you would like to project?
Wow, you just really hate plants, don't you?
1 & 2) Alright, not all of them do, but a lot of them do. His point is correct; without them, there would be no oxygen, we would die.
3) They may not be the ones that provide us food, but there are a lot of plants that provide us food. This is an obvious attempt to point out a non-existent flaw in his reasoning.
4) Ha ha, very funny.
5) You're a troll.