Can Historian data be exposed via OPC?

As far as I know, there is no first party solution. Historian data can be exposed via SuiteLink or PCS.
There are 3rd party solutions on the market to achive this though.

It depends on what kind of data you want to expose via OPC. The OI Gateway will serve "latest value" via OPC-DA (but as of April 2024, not OPC-UA), but will not expose historical data or alarm history. 

It also depends on what you are trying to achieve with the client: many OPC clients needing historical data also support SQL and REST interfaces for that, which can work with Historian directly. In most cases the SQL interface will be more flexible than the OPC-UA one. 

Thanks for your response, Elliott. Are there any documents available on exposing the latest value via OPC-UA using the OI-Gateway as mentioned?

My original reply above was mistaken. I've corrected it above (changes in blue). However, adding the OPC-UA server support to the Gateway (my original answer) is something under consideration for the next major release.