Immediate reaction: oh thank god, I remembered correctly.
Oathbreakers was always my favorite of the duology, and now I remember why. It's far less episodic than Oathbound, with a coherent plot for all but the first few chapters of the book, and of course there is much, much less rape, all of which is offscreen and specifically noted as a power crime over everything else. It's actually really interesting how it's treated in Oathbreakers as opposed to Oathbound: Oathbound it's treated as kind of inevitable, which is really upsetting, but this book notes that it's so often used against women in an attempt to break them or punish them, which I quite liked, actually. Sort of Mercedes Lackey calling herself out, though I'm not sure if it stuck quite yet. Her later books don't have anywhere near as much sexual violence as the first few did, though, so she did learn.
That aside. Oathbreakers continues the story of Tarma and Kethry several years after Oathbound left off. They've become professional mercenaries and joined a company, the Sunhawks. The book picks up in the middle of a war, and we deal with that for two chapters before the real plot of the book kicks off in chapter three. Their captain, Idra, is actually a princess who renounced her right to the throne, and now her brothers are squabbling over it and she has to return home to settle the squabble. Then she vanishes. Tarma and Kethry head north to Rethwallen, Idra's home country, to find out what's become of their captain. Spoiler: it's nothing good.
Along the way they hook up with Jadrek (not literally-- at least, not in Tarma's case), the Archivist of Rethwallen. He's got knowledge that is incredibly useful for them, and what I think is rheumatoid arthritis, not that it's super important to his plot arc apart from background and self-esteem issues. Also, relevant point: it's disabling, and while he gets some better palliative measures throughout the book, he's never cured. When he turns up in later stories he's still disabled, and I think that might be pretty important in terms of representation. I am not physically disabled so I don't want to speak for that community, but I appreciated it. Anyway. He ends up being a close ally and falling in love with Kethry, and there's some mutual pining for a while. This may have been the book to establish my love of mutual pining, come to think of it-- either this or Arrow's Fall, but I can't remember which I read first. Damn you, Mercedes Lackey, you established so many of my ironclad story kinks. Sigh.
I think Oathbreakers is much better than Oathbound, and I also think it's possible to read Oathbreakers without having read Oathbound, and enjoy it too. So if you're planning on reading one of these books, just stick with Oathbreakers, it's much better.
And there's less sexual violence. Always a good thing.