While there is no single ISO, Microsoft does provide an Offline Installer for Microsoft 365. This is the closest you will get to an ISO. You can download a folder containing all the installation files (roughly 4-5 GB) to a USB drive or network share.
How to get the Official Offline Installer:
Sign into the Microsoft 365 Admin Center (admin.microsoft.com). Go to Show All > Settings > Org settings > Office software download settings. Enable "Offline installation" to generate your deployment share. ms office 365 iso
Note: This applies only to Office LTSC 2021, Office 2019, or Project/Visio.
Step 1: Download the Office Deployment Tool While there is no single ISO, Microsoft does
Step 2: Extract and Configure
Step 3: Create a Configuration XML
Step 4: Download the Source Files
Step 5: Create the ISO