For 2018: To log out, click on back to account, which is at the top right of the screen. This will take you back into the OLT ONE account. Then you will find a little person icon at the top right of the screen. Click on this and then choose log out.
For 2019, 2020, and 2021: There is a log out option in the left margin of the tax year screen.(last tab in list)