The reading is pretty weird, but then again so are people from Osaka (just kidding, Osakazens!). The 大 reads like おお from おおきい. Then, 阪 uses the kun'yomi reading as well, which you didn't learn. That being said, doesn't this kanji look like 坂, which also means slope? Well, it shares a kun'yomi reading as well, so if you've seen 坂 you can apply that reading over to here, making おおさか. Then again, if you know this word means Osaka you probably can figure out how to write it too. Don't forget the double おお though!