I have a 78K OBC rank in JEE Main. Can I get admission to BTech CSE at BITS Pilani?

Unlike government institutes like IITs and NITs, BIST Pilani does not offer reservations based on caste. Also, the JEE Main exam is not considered by BITS Pilani and it only allows admission based on its own entrance exam called BITSAT. The exam is 3 hours long and will test your proficiency in physics, chemistry, mathematics and English along with logical reasoning. 

Coming to your second concern, BITS Pilani does not offer any BTech courses. As you are interested in CSE, you can consider the BE course course with Computer Science specialisation. 

BE Computer Science at BITS Pilani will provide you with an understanding of computer science principles and technologies. Hands-on projects and internships will add to the theoretical learning fostering practical skills required for success in the field. You will be exposed to an enriching experience readying you for various roles in the computer science field.