In Oberon's speech found in Act 2, Scene 1, lines 253-272, we find several themes.
The first theme is tricks and deception. Oberon is telling Puck he plans to find Titania sleeping on a bank where she usually sleeps and squeeze the juice of the magic flower onto her eyes so that she falls in love with the first thing she sees.
A second theme is fantasies vs. reality. Oberon is telling Puck to use the juice of the flower and "make her full of hateful fantasies" (263). If Titania falls in love with the first hideous creature she sees, just as Oberon plans, then Titania's most horrible fantasies will come true, which will merge fantasy with reality.
A third theme we can see is jealousy. We know that the reason Oberon is playing this horrible trick on his wife is that he is jealous of the Indian boy she is caring for, even wanting her affection all to himself rather than sharing it with a boy. In addition, a fourth theme is revenge. Oberon is avenging himself on Titania for refusing to give up the Indian boy.
A final theme we see in this passage is the desire to amend. We see this theme in Oberon's command to Puck telling him to enchant Demetrius's eyes. Regardless of the seemingly heartless trick Oberon is about to play on his wife, Oberon is not all bad and feels genuine compassion for Helena. He wants to mend Helena's broken heart by enchanting Demetrius so that he is now giving Helena double the love Helena is giving him, rather than the hatred Oberon has just witnessed, as we see in Oberon's lines:
Do it when the next thing he espies
May be the lady ...
Effect it with some care, that he may prove
More fond on her than she upon her love. (II.i.267-270)