We have a Roku integrated smart TV. We placed an over-the-air antenna on the window, which is capable of picking up over 40 local channels. We can watch the OTA channels using Roku by simply clicking on the Antenna tile. However, when we connect a fireTV stick 4K on the same TV we can't figure out how to watch the OTA channels on the firestick. I don't see a similar "tile" with the firestick . If not possible, is there a free App like Pluto that will allow me to watch OTA's ? Using Roku, Pluto does show the 1st 15 OTA channels that we get with the OTA antenna. Any help would be appreciated.
You are not watching ova channels with Roku. You are using the ova tuner built into the TV not built into Roku.
Since the Firestick doesn't have it's own ova tuner built in you have to use an app to do live TV. You can find some options here https://www.firesticktricks.com/amazon-fire-stick-apps.html
but I don't think it will replace using your antenna.