Should I go for coaching and drop 1 year after 12th to crack the OUAT entrance exam?

That depends entirely on the candidate. It is recommended that you prepare for your 12th exam along with other entrance exams. Try joining coaching centres that will properly guide you with subjects like Physics, Chemistry, Mathematics or Biology.

Because it is not justified to take coaching only for getting into OUAT, as to qualify you will require to be thorough with your concepts, quantitative aptitude and learn about basics of the science subjects. 

If you work hard enough, dedicating most of your time in the preparation of the exam, you will be able to sit for any entrance exam and score good marks.

If you are interested in agriculture, there are 2 ways you can join OUAT. One through OUAT entrance exam and 2nd by ICAR entrance exam.