OSD stands for Optional Standard Deduction. Instead of tracking all your expenses to get your net expense, with OSD, you can simply declare that 40% of your income is your expense. That means 60% of your income is taxable.
An added advantage of using OSD is that you don't need to file any audited financial statements. If you want to learn more about whether you need to file an FS or not, you can read this article: Do I need an Audited Financial Statement (AFS)?