How do I save myself from fake FIR/case?

If you have got a genuine OBC certificate from the competent authority then there is nothing for you to worry. Let them go and check the authenticity of your OBC certificate and get convinced that it is genuine and it would be summarised that some person is just trying to trouble you. In fact once that person is found to trouble you in a fake manner then he is also to be prosecuted in the matter and you can if you so wish lodge another FIR in the police about the mal fide intentions of that person to create problems for you.
What I will suggest is that you lodge a counter FIR against the person and give all the details of the talks between you and him. If you have available then give the Whatsapp chat also if by chance any of it was there and still available in your phone.
Apart from that write one application to the District Magistrate and Police Commissioner in the area telling them that such and such threat was given to you and you are feeling unsafe and unsecure from that bad element and in order to save the society from the ill doings of such people the administration should take up some action and that person should be taken in the custody or you may be provided some security guards.
If you present your case in a correct manner and approach the higher ups then the mischievous person would leave the scene to safeguard himself. Take help of some senior members of your family or relatives or even a lawyer in the matter and do not worry because he cannot do any harm to you. Once he is trapped in the process that I have suggested he would start asking forgiveness in the matter. Please keep the higher ups in the loop. Even approaching a MLA or a MP would not be out of context here.

This type of problem is increasing and one should be careful with such people. That is why when we work with other people we should not talk about caste, religion etc. We should work and talk about work only.
If any SC/ST person is filing a complaint at the police station saying that they are being harassed based on their caste, the police have to take immediate action.
I think it is better to talk to the other person and ask him why he has filed an FIR. But you should not talk to him alone. You should take a person whom both of you know. I think your boss will be the best. If he can call him and make both of you sit together and sort out the problems. After that, you can ask him to withdraw the case. That is the best solution.
If it is not possible, the second way is to go to a lawyer and ask him the way out. He may guide you properly. He may take you to the police station and make you talk to the officers there and take a petition from you also. Then the police may call the other person also and try to do something on that issue. As you are an OBC candidate, the lawyer may suggest some way of settlement with the other person.
If you know any well-known politician in your area, he may be an MP or MLA or any other person, you can take his help and manage the other person. SC/St atrocity case is a very serious case and police.

The situation with which you are facing is not unusual in our societies due to prevailing jealousy of someone seeing in the comfortable position. Now how to counteract this ugly situation would largely depend upon your initiatives. Throughout this process, you need not be over sentimental and proceed with the cool manner. I think the following steps could reduce your issue considerably.
1) Since both you and your colleague harassing you and even has lodged a FIR against you are working under the same boss. You might approach your boss to intervene in this case to diffuse the situation with his convincing talks but before proceeding, you have to take your boss in confidence telling him the entire situation so that your boss could make out the real happenings. In case, he is ready to help you with his active talks, allow him to do so.
2) Don't go publicly for the wide publicity of your unfavourable situations and in that way, you are inviting others to make your case difficult.
3) Take the help of a reputed lawyer in resolving your present impasse and follow sincerely whatever he says lawfully. If you have a recorded talk/ talks between you and the colleague harassing you, it could be still better.
4) If the situation demands your safety, you might take the help of the police officer with your visit in the police station apprising the officer of such a step for his unpredictable behaviour leading to endangering of your life. It could release you from this troubled situation.
5) Take the help of your local MLA or an influencing personality including an MP to resolve the case amicably.
6) I hope you would try your best not to outburst your emotions with your harassing colleague to make this case more complicated.