Hello Syam Dileep(C),
Since Outlook version 5.2637.0 crashes immediately on iOS 27.0, while other Microsoft applications and Outlook on the web continue to work, this behavior points to a compatibility issue between the current Outlook mobile build and the new operating system environment.
At this stage, the issue will need to be addressed through an update to the Outlook for iOS app. Please keep Outlook and iOS updated and install the next available Outlook release when it becomes available.
In the meantime, please ask your IT administrator, or someone with access to the Microsoft 365 Admin Center to report the issue by going to Health > Service health > Report an issue and submit the Outlook version, iOS version, device model, crash behavior, and the troubleshooting already completed.

The administrator can also use Help & support in the admin center to open a support request with Microsoft.
As a temporary workaround, you can continue accessing Outlook on your iPhone through Safari at https://outlook.office.com while waiting for an Outlook update.
Please also keep Outlook updated through the App Store so that the fix can be installed as soon as it becomes available.
Kind regards,