Within Assets Folder // fonts 
Incl fonts within folder

npm install expo-font if not already installed
import * as Font from "expo-font"

const fetchFonts = () => {
  Font.loadAsync({
    'open-sans': require("FILE-LOCATION")
    "open-sans-Bold": require("FILE-LOCATION")
  })
}

To ensure fonts have loaded when app starts - 
  const [ fontLoaded, setFontLoaded] = useState(false);

if(!dataLoaded)
  return(
  <AppLoading
    startAsync={fetchFonts}
  	onFinish={() => setDataIsLoaded(true)}
    onError={(err) => console.log(err)}
  )
  
 Then in styleSheet --

fontFamily: "open-sans-bold"