Megan O'Hara was a chiropractor, breast cancer survivor, and patient of McNamara/Troy.
Megan had gone through both radiation and chemotherapy before having a double mastectomy. Leaving her with extesive scarring on her chest and no nipples. She didn't undergo any reconstruction following her surgery since she was already so physically weak from her treatments, and because one of her friends had a painful recovery after getting implants. O'Hara's illness caused a rift between her and her husband, so she decided to get reconstruction, and implants that were bigger than her original breast size.
She cancelled her surgery, and told Sean that she had left her husband since he no longer saw her as a "whole" person anymore. As Sean walked her to her car, they briefly kissed. The two began an affair shortly after when Sean came to see her at work when he started to have neck pains.
Megan then decided to undergo the surgery since she had fallen in love with Sean, and wanted to be physically intimate with him without feeling self-conscious.
Sean eventually tries to break off the affair, after his son Matt finds out about it. However, this happens at the same time that Megan finds out her cancer has returned, and that it's terminal. She decides that she would rather commit suicide than go through a slow, painfull death. She asks Sean to help her go out in a peaceful way. Despite his reservations, Sean decides to help carry out her wish. They rent a hotel room where Megan takes pills given to her by Sean that will cause her to overdose, and put's a plastic bag over her head in order suffocate in her sleep. Megan's funeral takes place by the beach where her ashes are scattered into the ocean by her friends and family.
In an episode where Julia dreams of what her life could have been like if she hadn't married Sean, she imagines that he and Megan are happily married with a son, and pregnant with a baby girl. Julia explains that since she never met Megan, she always wondered what might have happened with her and Sean if she hadn't died.