Oppo R5 comes with Android 4.4.4 Kitkat OS. It runs on ColorOS screen. This device is one of the most slimmest model. Rightow it is hard to find a good custom rom for this device. But still you can use root the phone and test things on it. It is possible to root the phone through ForceRoot Apk method. I found this file Oppo Forums. There is no other effective method of rooting. It also not 100% sure that this apk file is completely secure to use. I am just providing the steps her. The process is below and do this on your own risk.
First Download ForceRoot APK file. You can find this easily on Google. There is one file available on Oppo forums shared by a user. You can download that from here. Once you are done with downloading the file, you have to install this in your device. You have to provide some special permission for the same.
Tap on Root. The app needs OppoTools Plugin. You have to check that your device must be connected to internet. Allow the installation.
After the installation you will find a option as One Key Root. Tap on it. The device will reboot after sometime. After that allow SuperSu permission for final process.
To reverify the root status you can install Root Checker from Google Play and see if it is properly rooted or not. That's it. The device is not rooted and you can do many things. For example you can take the full rom backup using different android software.