It's important to note that Microsoft 365 E3 is not the same as Office 365 E3 - they are completely separate products.
M365 E3 includes all the features of O365 E3, plus the OS (Win10 Enterprise), AAD premium, InTune and some additional security features.
O365 E3 is £17.60 pu/pm and M365 E3 is £28.10 pu/pm
If you've only got O365 E3, then no, you still need the separate OS license.
If you've got M365 E3, then I can't see why you couldn't get rid of the annual Win 10 Enterprise license. But as with all things relating to MS licensing, I'd advise you speak to your reseller!

@scottalanmiller My understanding is that as soon as a user is authenticated, they require a CAL.

We have a web app sat on a Windows server which hooks up to an Oracle database. Our staff login to the site to collect / process their jobs for the day and since we authenticate them in Oracle, they need a Windows Server CAL.

This excerpt makes it pretty clear...

"If you have Windows Servers configured to run a “web workload” these users will not require CALs or External Connectors. However, let’s say you are using Windows Server to setup an online store where customers can buy widgets. You have front end Windows Servers setup to support your website, and backend servers (e.g. commerce servers) setup so customers can check out and buy your widgets. The front end servers used to host your website would generally be considered as running “web workloads” and CALs or External Connectors will not be required to access these servers. Once the customer adds a widget to their shopping cart, creates an account and enters their credit card and shipping information to complete the sale – they are now authenticated via your back end commerce servers/application (non-web workload). Since users are accessing the backend commerce servers which web workloads are not running – CALs or External Connectors will be required for users to access these back end servers."

It's important to note that Microsoft 365 E3 is not the same as Office 365 E3 - they are completely separate products.
M365 E3 includes all the features of O365 E3, plus the OS (Win10 Enterprise), AAD premium, InTune and some additional security features.
O365 E3 is £17.60 pu/pm and M365 E3 is £28.10 pu/pm
If you've only got O365 E3, then no, you still need the separate OS license.
If you've got M365 E3, then I can't see why you couldn't get rid of the annual Win 10 Enterprise license. But as with all things relating to MS licensing, I'd advise you speak to your reseller!

@tim_g Interesting - I hadn't spotted that key distinction!

@scottalanmiller Exactly - there are various other factors too. In our particular situation, we would need CALs anyway as our site isn't publicly accessible but I was just making the point.

@scottalanmiller My understanding is that as soon as a user is authenticated, they require a CAL.

We have a web app sat on a Windows server which hooks up to an Oracle database. Our staff login to the site to collect / process their jobs for the day and since we authenticate them in Oracle, they need a Windows Server CAL.

This excerpt makes it pretty clear...

"If you have Windows Servers configured to run a “web workload” these users will not require CALs or External Connectors. However, let’s say you are using Windows Server to setup an online store where customers can buy widgets. You have front end Windows Servers setup to support your website, and backend servers (e.g. commerce servers) setup so customers can check out and buy your widgets. The front end servers used to host your website would generally be considered as running “web workloads” and CALs or External Connectors will not be required to access these servers. Once the customer adds a widget to their shopping cart, creates an account and enters their credit card and shipping information to complete the sale – they are now authenticated via your back end commerce servers/application (non-web workload). Since users are accessing the backend commerce servers which web workloads are not running – CALs or External Connectors will be required for users to access these back end servers."