The steps are as follows:

1. Open the app store on the OPPO phone: Find and click the "App Store" icon on the home screen.

2. Search for ringtone download applications: Enter "ringtone download" in the search bar of the app store, and then click the search button.

3. Install the ringtone download application: Find an application named "OPPO Mobile Ringtone Download" in the search results, and click the install button to download and install.

4. Open the ringtone download application: After the installation is completed, find and click the "OPPO ringtone download" application icon in the app store or on the home screen to open the application.

5. Browse free ringtone resources: In the ringtone download application, you can browse and select many free ringtone resources. You can find your favorite ringtones based on different categories, rankings or search functions.

6. Download ringtones: After finding the ringtone you like, click the download button to download it. After the download is completed, the ringtone will be saved to your OPPO phone.

7. Set a new ringtone: After completing the download, return to the home screen of the OPPO phone, find and click the "Settings" icon. On the Settings page, select the "Sounds & Vibration" option, then click "Mobile Ringtone."

8. Set ringtones: In the mobile phone ringtone settings page, you can see the list of downloaded ringtones. Select the ringtone you want to set and click OK or Save button.