Dear Joya, 

Unless stated by the ONGC, you do not have to pay the counselling amount of 50,000 as you are not going to appear for the counselling in the first place. So, technically there will never come a time when you will be prompted to pay this amount. 

It is the rank of CLAT of a year that matters and that rank will remain with you whether or not you pay the amount. 

COming to ONGC, you should keep yourself updated about that and fill its recruitment application form. You will have to apply to ONGC as per the process to be uploaded at ongcindia.com with the relevant link. You can also check the careers360 link given below to check the same:

https://law.careers360.com/articles/ongc-recruitment-through-clat

I hope it helps. 

All the best!