Marshmallow 6.0.1 OTA Update - Boot Loop

Samsung A3 Model: A300FU Country: United Kingdom

My phone was previously locked on O2 and my brother got it unlocked and i was able to use a Virgin sim. Yesterday I updated the phone's software to the latest Marshmallow 6.0.1 using OTA but, now the phone is stuck on SAMSUNG LOGO for an indefinite period of time. I tried: clearing the cache data multiple times but that did not work, flashed LOLIPOP firmware but that too was stuck on SAMSUNG Logo. Currently i flashed the firmware back to Marshmallow 6.0.1 which was on O2.

When i go to recovery mode i see "O2U CSC code applied" What does this mean?

My questions are: 1) Since i flashed a firmware which is on the carrier O2, is my sim now locked on 02 even though it has been unlocked before? 2) How can i come out from the boot loop without clearing all my data? 3) Will clearing data/factory reset resolve the problem?

Thanks