Hey, this is a very general problem. In these cases,
Banks deduct TDS on interest as and when interest is accrued, not when interest is paid out. This TDS deducted reflects in your Form 26AS automatically. Hence to prevent confusion in Form 26AS, it is advisable to pay the self assessment tax on your interest income (if applicable) on a yearly basis, and not when the FD matures.
Answer to your query, when you declare interest income as a lump sum amount when your FD matures, there is a possibility that you will be included up to a higher tax slab and pay higher taxes overall. To prevent this, simply pay the tax accrued on your interest income on an annual basis. So, if TDS is not deducted by the bank, take the amount of interest on accrual basis and pay tax accordingly.
On the point of income under OS, mention income accordingly as told above. There is no tax on principal amount, hence 50,000 can never be a part of income under OS.
Cheers !