The biggest problem with Oadenol's Codex is that it was published at the wrong time. As I mentioned in my Wonders of the Lost Age review, publishing that book first set the tone of the Exalted line toward robots and sentai teams and airships and the best forms of magic being indistinguishable from technology. It would have been a lot better to publish Oadenol's Codex first, since it's more than half-full of material that mortals can accomplish and would thus set a better baseline of expectations for Exalted. I'd much rather have had later material written assuming a world of priests putting up prayer strips to ward off hungry ghosts and rites sung to bring in the crops and architects spending half their lives building precisely-calibrated mansions to contain the updwelling of Essence from a dragon line and cautious expeditions into the Wyld to retrieve powerful ingredients. Then assume the Exalted get to break the rules, the same way the Exalted corebook has rules for bleeding and infection and fatigue and months-long convalescence after battle and gives the Exalted a way to ignore all of that.
There are four main parts of the book, and three of them are easily accessible to mortals. My favorite part is definitely the thaumaturgy section, which has rituals from alchemical mixes to controlling to weather to calling up the dead or demons to breeding better animals. There's also a note that thaumaturgy draws on the underlying processes used by the gods to maintain Creation, which I think is a great baseline. Really interesting worlds can be created out of assuming that natural processes aren't simply bound by real-world physics, as Glorantha has spent decades demonstrating. The example, of the little god of a hunk of clay performing the Mudra of Isolate Stability when entering the furnance in response to the fire god's Song of Ardent Unity, thus producing a brick, is a bit silly, though. It's like how the least god proliferation leads to gods being by far the most common form of life in Creation, since every blade of grass and grain of sand has one.
The rules for thaumaturgy are not quite as good. It's Attribute + Ability vs Difficulty like everything else, but the Ability is almost always Occult and often other Abilities are more obvious. For example, Alchemy requires Craft (Water) to know the formulae, but even though Craft (Water) is the skill for making potions and liquids of all kinds, it's not rolled. Geomancy doesn't require Craft (Earth). Enchanting doesn't require any Craft at all. Spirit-Beckoning uses Occult even though the skill for prayers is Performance and most of the individual rituals override the base and say they're rolled with Charisma + Performance. The way thaumaturgy is written up with how fundamental it is to Creation's functioning, it should have been just been an extension of mundane skill, but instead it's halfway between that and being its own thing and suffers a bit for it.
The manse and artifact construction rules are pretty good, with some great examples of demesnes, manses, and all the non-technological artifacts that were missing from Wonders of the Lost Age. There's also an important note that artifacts should do something special and not just provide mundane bonuses--a hammer that adds +2 to Craft (Fire) rolls is just a very well-made hammer. The major problem I have with these rules is that the tech fetishism of Exalted 2e comes to the fore again in the terminology of features that can be built into Manses. Names like "Central Control" "Self-Destruct Sequence" or "Network Node" just perpetuate the idea that sufficiently advanced magic is technological without room for other paradigms.
Also, that focus led to a fluff/rules split. Having a Repair rating or requirement for Maintenance means that the artifact-maker or manse-builder gains extra points toward giving powers to their construction...but that means that the imperishable wonders of the First Age Solars, which work flawlessly for millennia with no care, should be less powerful than the Shogunate wonders made by the Dragon-Blooded. And since that goes against all the themes of the game, it seems like a bad design decision.
There was a throw-away line in the thaumaturgy chapter that I liked:
"The reptiles suffered tremendously during the war, with whole nations exterminated; yet, they might have recovered if they had kept the gods' favor."
That makes me wonder what Creation would have been like in that case. If the Unconquered Sun had kept his Dragon King form, and the Dragon Kings had kept the Mandate of Heaven, what would have happened to the Exalted? Would they have been second-class citizens in a world of subjugated humans? Would they have risen up again and overthrown the gods? There's a lot of great stories there.
There's a chapter in the end about magical flora and fauna of Creation, but I think it suffers a bit by being mostly conversions from first edition. We've seen ink monkeys and heart wasps and ironwood before, and while getting their second edition stats is nice, it would have been nicer to see something new.
Other than the flaws relating to drawing on Wonders of the Lost Age for its background, Oadenol's Codex is a great book. With its focus on a lower-level of the Exalted world, it builds a foundation for Creation that I really like with only a few missteps.