The amazon.com product description:
Tarma witnessed her clan's murder and, swearing vengeance, became a master warrior. Kethry fled her forced "marriage" and became an adept--pledging her power to the greatest good. When Kethry obtains a magical sword which draws her to others in need, the two vow to avenge the wrongs done to womanhood.
That first story is really key to understanding these two characters, although enough of it is sketched out through the course of Oathbound that you can pick up on their background.
If you're familiar with the Sword and Sorceress series, one of the chapters in this book will seem very familiar to you - Chapter 6, which was published as a story on it's own in Sword and Sorceress V. It's the story based around the song Threes, and I'm glad it was included in Oathbound as well as the climax of the book is based around the events of that story.
Oathbound and Oathbreakers were Mercedes Lackey's first real forays beyond the borders of Valdemar, fleshing out the world of Velgarth quite nicely, and they're two of her books that I've really enjoyed reading over the years. I certainly enjoyed re-reading Oathbound this time for the Valdemar Reading Challenge and the Hardcore Re-reading Challenge (although it wasn't on my initial proposed reading list for the second challenge).