These involve the sale or purchase of securities by the central bank to withdraw liquid funds from the banking system or inject the same into that system. OMO allows flexibility in terms of both the amount and timing of intervention, which did not exist in Bangladesh before 1990. Bangladesh Bank introduced a 91-day Bangladesh Bank Bill, a market-based tool for monetary intervention, in December 1990. The bank bill was subsequently withdrawn from the market. At present, OMO operations are conducted through participation of banks in monthly or fortnightly/weekly auctions of treasury bills.