<ModSquad: This critique was authorized by the creator of the MOC. It’s rough, but he has been warned of and consented to my verbal obliteration.>
Oh boy, where to start. Ground up?
I think we’ll start from the ground up.
FEET
…they’re feet? Really not sure what to say. I’ve always liked that particular foot piece, but the Technic addition here isn’t very well done. Looks too thin from the back and too blocky from the front. Otherwise…eh? Not really a fan of them.
LOWER LEGS
Super basic design that would be fine with a bit more work - not even specifically work on the lower legs. More on that in the torso section. Otherwise, they need some filling in and some smoothing out, maybe, but for what they are I guess they’re alright.
UPPER LEGS
Nopenopenopenopenopenope
nopeitynope
Really, really bad. Far too thin for the bulky bruiser design the rest goes for, and overall the combination of those parts stopped being “good” years ago when we got better alternatives. With some tweaks it could work, but I feel like it would be more hassle than the design is worth. All I can honestly recommend here is a redesign.
WAIST
Uhh…a little too hollow? Needs some filling out but otherwise this is fine. No need to over complicate something that doesn’t need to be.
TORSO
This is where the fun begins.
Because this is…uh…interesting?
I think the use of the Pakari Nuva as pecs is creative and interesting - and it’s not that part of the design that irks me.
First off, the Nuva shoulder in the middle of the chest needs to either go away or get flipped upward. As-is, it looks very flat and out of place compared to the armoring right beneath it.
Speaking of the armor right beneath it, the Mantax shell - while I commend the usage of such a fickle part - isn’t really used all that well in my opinion. It excels at manipulating visual weight, but here it just kinda makes him look thick sideways. Wouldn’t be an issue if the thighs were bulkier and the shoulders were narrower, but because of those two items not being fulfilled, the design falls short of optimal. The tubes on the front also look very random and distracting, so I’d personally recommending cleaning those out.
Additionally, he has one of the worst cases of a T-orso I can ever think of. Incredibly T shaped with very little transition. I’d recommend pulling the shoulders in quite a bit.
UPPER ARMS
Probably the best part of the MOC. Aside from the Metru thigh armor on top of the Hordika foot, you’ve got their flow down pretty well, and the armoring is done well enough. Aside from the aforementioned flow-breaking Metru thigh, I actually think I like it. Not much else to say.
LOWER ARMS
I like Piraka legs.
I do not like Piraka legs as arms. Ever.
Third least favorite part of the MOC at least. They work as legs, but here the design is far too sleek and backwards to properly suit as arms. Additionally, my Rule #2 is broken by both forward and backward facing armor. Custom arms will be your friend here. The hands (just gonna lump them in with the lower arms) are maybe a bit too prefab, but since this is supposedly Onua I can’t really complain too much about some killer Shredder Claws.
HEAD
Yes. You heard me. Head.
It doesn’t work.
The mask here is completely atrocious and contradictory to the character shaping in every way possible. It is incredibly sleek, flat, thin, and tall. The MOC itself is textured, greebled, bulky, and squat.
I could see Onua 2015’s mask working here, as the set in question shared very similar proportions to what you have here. Possibly even Onua 2016, but I don’t own that mask so I cannot confirm or deny. Almost any other mask would be better here, provided it was in the right colors.
CONCLUSION
Overall - not a fantastic MOC. I see quite a few areas for improvement though, which means potential. The only place I see no redemption for would be the thighs…just, start over for those maybe. Just my two cents at least.
Something I do commend it for, however, would be the color scheme. Generic, yes, but it’s consistent, and the blue pins don’t distract me too much. Perhaps on a sleeker character it would, but here the textures work in your favor for once.
Good luck, and keep on MOCing! You’ll get there 
If you see my request for information on how to post a new thread, needless to say you may disregard it now lol. Onto the topic: My Toa of Earth MOC. It is not necessarily a revamp of Onua specifically, and when I built it (over a period of two years off and on) I generally wanted it to be a generic Toa of Earth. As it progressed, it began to resemble Onua more and more, at least in my mind. This is easily one of my top three favorite MOCS I’ve built. My most favorite aspect of it is how solid the build is. I have tried many times to change or add armor to him, and I cannot find any reasonable ways to do so. Only with that in mind may I consider him complete! So, here are some pictures, and I absolutely encourage and appreciate all feedback and constructive criticism. Thank you for taking the time to check this out and I hope it inspires you to find new ways to build MOCS!