There are multiple ways to importing mass transactions into QuickBooks Online.
Import a Web Connect file from your financial institution
Many financial institutions offer the ability to download your transactions to a .QBO, .QFX, or .OFX file. These files can contain up to nine prior months of transactions. For more details, see upload more than 90 days of bank transactions through Web Connect.
Import an Excel file
Checks and deposits can be imported into QuickBooks Online via an Excel spreadsheet. For more details, see import bank transactions from Excel CSV file to QuickBooks Online.
Import from QuickBooks Desktop
In the current versions of QuickBooks Desktop, there is a batch entry feature. If you're just starting your QuickBooks Online subscription, you can enter your historical data by batch into QuickBooks Desktop prior to importing your file into QuickBooks Online.
Use a 3rd-party import tool (such as Transaction Pro Importer)
These 3rd-party tools are developed to provide features that aren't currently built into QuickBooks Online. For options, check out the Intuit Appcenter.