We might say that the thesis or main idea of Obasan by Joy Kogawa revolves around identity, secrets, pain, and healing. Let's look at these in more detail.
When Naomi is five years old, she is abused by a neighbor, which leaves her with a great deal of guilt and pain. Her life turns upside when her mother goes back to Japan, her father is taken to a work camp during World War II, and she and her brother, Stephen, are moved along with their aunt and uncle to perform hard labor in various places. This life takes a major toll on a young girl, and Naomi must suppress much just to survive mentally and physically.
Naomi has a hard time keeping track of who she is during this period. Her father presumably dies of tuberculosis, for Naomi never sees him again after one quick visit. Her mother does not return from Japan. Stephen leaves the family to pursue his music, and Naomi grows up to be a quiet woman who is not able to express her emotions.
Naomi does not begin to experience any true healing until she finds out what has really happened to her mother, whose absence has long affected Naomi. Naomi finds a letter that tells her mother's story. Her mother and grandmother were in Nagasaki when the atomic bomb was dropped, and they were both horribly injured. Naomi's mother was so disfigured that she could not bear to have her children see her. Naomi's aunt and uncle kept the secret for many years, even after Naomi's mother's death a few years after the war.
When Naomi finally understands what has happened, she can begin to let go of the pain. The secrets are revealed, and she can rediscover herself and begin to heal.