Other forms: overjoyed; overjoys
To overjoy is to fill someone with extreme happiness. If you adopt that fuzzy kitten and give it to your little sister, it will absolutely overjoy her.
Your teacher has the power to overjoy your whole class — all he has to do is cancel the math quiz and order pizza instead. And you can just as easily overjoy your grandparents by arranging a surprise reunion with all of your cousins. The adjective overjoyed (or "deliriously happy") is more common than the verb overjoy, but they're both great words for describing a thrilling, glorious happiness.