Well, looks like to dump boot.img you need root. Please try attached file (unzip it). Found it in TG, labeled as boot.img for indian Oxigen. Please note that you should NOT flash it to your device, so it should be safe enough. We will try to sideload it.
You already have installed fastboot to your PC, so I will skip this part.
To check boot.img
1. Place boot.img to your computer, to the same folder where you have fastboot installed
2. Reboot your phone to fastboot mode i.e with
adb reboot bootloader
3. Do
fastboot boot boot.img
.
If phone will start normally, looks like boot is ok and you can use it to get rooted.
To install Magisk (get root)
1. Place boot image to phone.
2. Download
Magisk and install it to the phone. Note that we use canary version of Magisk. Latest stable version doesn't work at least on chinese (ColorOS) version.
3. Start Magisk on a phone, press "Install" and choose "Select and Patch a File" (the only available option), select boot.img file.
4. Patched boot.img will be placed to Download directory named something like "magisk_patched*.img". Download this file back to PC, place to the same directory with fastboot.
5. Reboot your phone to fastboot mode i.e with
adb reboot bootloader
6. Do
fastboot boot magisk_patched*.img
(change to correct name of file). Phone should start correctly.
7. Start Magisk again. Select "Install". This time all options should be available. Select "Direct Install (Recommended)".
8. At the end reboot and check if root is working.