Yes, you can choose Odia as your language paper for the UPSC mains examination. Here are some recommended books that you can study for the Odia language paper:

1. "Odia Bhashara Byakarana" by Dr. Banamali Mishra: This book covers the grammar and syntax of the Odia language. It will help you understand the structure and usage of various parts of speech.

2. "Odia Sahitya Itihasa" by Dr. Pranakrushna Parija: This book provides an overview of the history of Odia literature. It covers the different periods and genres of Odia literature, giving you a comprehensive understanding of the subject.

3. "Odia Sahitya Ra Itihasa" by Dr. Dinabandhu Sahu: This book also delves into the history of Odia literature but provides a different perspective. It includes detailed chapters on the major literary figures and their contributions to Odia literature.

4. "Odia Sahitya Sanchaya" by Dr. Gopabandhu Das: This book is a collection of selected works of renowned Odia writers. It includes poems, short stories, and essays, allowing you to familiarize yourself with the different styles and themes of Odia literature.

5. "Odia Vyakarana" by Dr. Manoranjan Dash: This book focuses on the grammatical aspects of the Odia language. It provides explanations and examples of various grammatical rules, enabling you to enhance your language proficiency.

6. "Odia Bhasa O Sahitya" by Dr. Bijay Mishra: This book covers both the language and literature aspects of the Odia language. It includes chapters on grammar, vocabulary, and literary analysis, providing a comprehensive understanding of the subject.

7. "Odia Bhasa Bigyan" by Dr. Prabhas Chandra Dash: This book explores the scientific aspects of the Odia language. It covers topics like phonetics, phonology, morphology, and syntax, giving you a deeper understanding of the language structure.

By studying these books, you will be well-prepared for the Odia language paper in the UPSC mains examination. It is also recommended to practice writing essays, comprehension passages, and precis in Odia to improve your language skills. Good luck with your preparation!