Welcome to Android Central!
Open the app drawer (I don't have an Oppo, so I can't say for sure how it works on your phone, but most others, it's a swipe up from the bottom of the screen or a tap on an icon that looks like a grid of dots), then scroll through the list until you find the phone app (or dialer or whatever Oppo calls it). Press and hold it, then drag it to the homescreen of your choice.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See
this link for instructions on how to join Android Central.
