Found this useful OTA Update for First Generation Android One Devices . This OTA was captured from Micromax Android One Device.
Download and Install Instructions can be found here :MOD EDIT Link removed, please no promoting things that require follow or like things.. redirect traffic.
Flashable zip for all Android One Devices is added up in the same link! This is for First Generation Android One Devices!
Updates on Installing on Unlocked Bootloader with Custom recovery
I downloaded the OTA above for my phone model (Karbonn Sparkle V) . I had unlocked bootloader for my previous CM 12.1 ROM. So the procedure is simple, I rebooted to recovery (PhilZ Touch) and then formatted data, system and cache. I then proceeded to install the zip file which automatically installed the boot and system.
Now I'm running a fresh 6.0 Marshmallow build while still retaining my unlocked bootloader. All things are running fine. Now I'm going to setup my SD card as an internal memory, before which I need to reformat it as I had partitioned it to run Link2SD.
All the best lads!
Edit 1: Use Custom Recovery option, uncheck the recovery checkbox. Do not the install from zip option directly, I lost my custom recovery that way. Re-installing recovery now.
Edit 2: I advise doing it the plain old, lock your bootloader, flash boring stock rom way or the phone will cause issue with the internal memory.