One of the least interesting stories ever told in any medium.

This review contains mild spoilers, but if it being spoiled keeps you from reading the manga, thank your good luck.
Our Blood Oath reads as though the author has read Tokyo Ghoul, watched a copy-paste vampire movie from the early 2000's (you know the type), and never consumed any other story, but has seen random pages of successful manga. It's one of those manga that does next to nothing original, but still manages to take well-established tropes and a copy-paste story and completely mess them up.
Let's start with the premise. Our Blood Oath is a series about two brothers, a vampire and a dude who's bound to the vampire through a blood contract. The latter is adopted. They go around beating the living daylights out of various supernatural creatures to protect humans. It's a pretty basic shonen premise, in line with the thousand other battle shonen out there. I have no problem with a basic shonen premise- executed well, it can lead to a very fun manga, such as My Hero Academia. The issue is that this series doesn't do pretty much anything original or interesting.
The first big issue lies with the characters. The main duo are stereotypical big-hearted lug and cynical child. Neither of them really has a personality besides "recycled shonen trope". The side cast is also boring and extremely bland- you have a dude who's swept up in supernatural issues who gets surprised really easily sometimes and other times doesn't bat an eyelash at supernatural stuff, who I'm pretty sure exists mostly so that the author can explain the plot by having him be clueless. Other than that, you have they-exist-I-guess kinds of characters, you know, the background kind that pop in for a few panels. Even the characters that I think were supposed to appear some more didn't, because this manga was rightfully axed at 2 volumes.
But wait, let's talk the villains! Honestly, there's next to nothing to talk about. The villains are entirely forgettable, don't really have interesting powers, and announce themselves with a stereotypical killing-dudes-in-an-alleyway scene, followed by forcing a kid to kill another kid. In all honesty, the latter was probably the most adventurous thing that this manga ever did. After spending a couple chapters "building up" the villains (look, they kill people, must be bad guys!), Our Blood Oath gets into the final battle and the villains lose all of the shreds of personality they had before.
The second big issue is the utter lack of any sort of real direction in the story. I won't entirely blame the author here, as that's partly the editor's job (the editor for Our Blood Oath also has an illustrious list of six other axed manga under his belt, including Protect Me Shugomaru!, Red Sprite and Bozebeats). Still, the story has absolutely nothing aside from being awful distinguishing it from a bunch of pieces of other manga mixed together. The basic plot is, of course, the brothers trying to keep people safe from supernatural creatures, but in the same vein as Tokyo Ghoul or Jujutsu Kaisen, there's other supernaturals who just see humans as disposable, yadda yadda. They fight. It's a very boring fight that equates to just kind of tossing blood around until one character powers up a stupid amount with the POWER of EMOTION and easily wins. The villains then cry and reveal their tragic backstories, which makes them sink even lower from boring villain to boring villain who also has a boring backstory (the typical "my family!... so I became evil!). Other than the villains, the story is just kind of rambling. I suppose it was trying to flesh out its characters or worldbuild, but both the characters and the world are bland as all get out and the first few chapters kind of reenforce that. Oh, but on the plus side, the author does some extremely cleaver and unique stuff for worldbuilding, such as a candy shop... with a kid that calls the owner in... run by a mentor-style character... selling supernatural stuff... with a secret cave basement. Yeah, if I haven't said it enough, this series is not original, and yes, I think this is a shameless rip off of that other Jump manga that was actually successful.
Last big issue, the ending. Personally, when it comes to axed series, I consider the ending few chapters to be the most crucial part- how well an author can finish up a story fast is on display, as is their ability to write in general. There are axed series that I like a lot mostly due to their endings, such as Red Hood's absolutely wonderful trip and a half or Time Paradox Ghostwriter not feeling like it was axed. Our Blood Oath, on the other hand, decides to toss a "shocking plot twist!" in that I'm sure would have worked better if the author could write well enough to keep the manga running for more than a couple volumes. Then, in copy-paste, badly written shonen fashion, an emotional conversation that has absolutely zero place in the series saves the day and everyone is happy again! The villains decide being villains is too much work (not actually, that'd be too humorous for Our Blood Oath) and the brothers are all happy in the sunlight again, nobody gives a damn about anything the villains did, and the author does a really good job of making sure the readers don't give a damn about anything the series did.
Art- I'll keep this short. The art is neither good nor bad. It's a little better than most failed series, but the layouts are uncreative, as are the character designs. The most exciting part of a character design is a character having multiple colors in their hair, in all honesty. The monster designs are possibly the best part of the art design-wise, but even then one is a pretty typical beast, one is Kurama but with three heads, and the vampires are weak body horror. I can't really fault it since Our Blood Oath ran in a magazine aimed at a relatively young readerbase, but it would have been nice if the monsters were scarier looking. That would have given them some impact, since all of them were heavily reliant on "look kids, murder!" to be at all threatening.
Ultimately, Our Blood Oath is an utter failure of a series in every sense. Fun fact, in the Weekly Shonen Jump ToC (which is roughly decided by votes of the readers of Jump), Our Blood Oath consistently did worse than such series as Agravity Boys, Hard-Boiled Cop & Dolphin, Phantom Seer (which is a huge upgrade over Our Blood Oath in the basic supernatural shonen department), and Moriking, What do those series have in common? They were all axed pretty fast. Yeah, Our Blood Oath was just that bad, even to readers of a magazine known for basic battle shonen. Got a bit off course there, but the point is that the series was genuinely awful. I didn't enjoy it in the slightest (and yes, I do normally enjoy basic battle shonen, this one is just awful), the characters were flat, the conflicts pointless. I don't see the author ever successfully writing a manga, though I could see them pairing up with a writer since the art is still passable. I gave the series the lowest score possible and I stand by that; it truly doesn't have redeeming features and reads like basic plotlines were thrown in a blender. If you have an interest in reading axed supernatural Shonen Jump manga, Red Hood, Ayashimon or even a bad series like Red Sprite will be infinitely better. If you're interested in Our Blood Oath for some other unknowable reason, I suggest watching paint dry instead.