Dipali, See, it is very important to go through the previous years and sample question papers in the form of books like Oswal and then analyse them to know about the important topics and the pattern of exam but you can't just rely on them for good marks, it may only be sufficient to get you decent marks except the case when you have good understanding of all the theory. Rather, your focus should be on understanding the theory with appropriate basics and concepts as the chemistry board question paper  is rather easy but requires proper understanding of the topic so that you can analyse the question in the exam and answer it based on your understanding of the topic. Actually, full understanding of the NCERT theory and the exercise questions are sufficient for the Chemistry exam to get you above 60 marks but going through the Oswal book would increase your chances to get probably 65+ out of 70 in boards, however , in case your theory is strong and you have understood all the concepts and basics including the reactions, mechanisms and conversions properly, then the NCERT is sufficient.

So, opt for sample papers only if you would finished the NCERT at least twice which is important according to many toppers for securing 65+ marks and you can't just rely on the sample papers for full marks.

Good luck for your examination.