Ahsoka Tano, a key character in the Star Wars universe, is noticeably absent in the original trilogy. The real-life reason for this is that she hadnt been created yet for the Clone Wars series. However, there are several theories and explanations about what Ahsoka was doing during the original trilogy based on her story in other Star Wars media. Here are some possible explanations:

• After her duel with Darth Vader, Ahsoka disappeared into the ruins of a Sith temple, leaving her whereabouts unknown during the Galactic Civil War.
• Ahsoka operated as "Fulcrum" and played a crucial role in coordinating rebel cells, eventually leading to the formation of the Rebel Alliance.
• Ahsoka spent some time living in the Coruscant Underworld and aided the Republic in the Siege of Mandalore.
• Ahsoka survived Order 66 by faking her death and going into hiding.
• Ahsoka joined Jedi Kanan Jarrus and Ezra Bridger on a mission to an ancient Sith Temple on the planet Malachor.
• Ahsoka was training Sabine Wren during the original trilogy era.

Its worth noting that some of these explanations are based on speculation and theories, and the upcoming Ahsoka series on Disney+ may provide more insight into what Ahsoka was doing during the original trilogy.