If you try to make a bread exclusively or almost exclusively out of oats, you are missing good gluten formation. Oats are gluten-free(1), so won‘t be able to develop the network that traps the CO2 from your yeast like a wheat bread does and what you made was (slightly sloppily phrased) baked oatmeal. Most bakers will tell you that you need a minimum percentage of wheat (or wheat relative like spelt) for a good bread. That’s not true, as some bread traditions that are based on rye have shown. Oat is not a classic bread grain, though.
Now, it would be too easy to simply say „you can‘t make bread out of oats alone“. It is indeed possible, but you need to use a few tricks along the way - and accept a few differences. I am basing this answer largely on the experiment of one German baker and blogger that I usually trust, so I will be paraphrasing the core findings in this post. He used an oat sourdough, a pre-soak/autolyse step for 1/5th of the oats and a comparatively low amount of yeast (which is probably more of his trademark than essential). Overall, he works with a hydration of 125%, which is lower than your 140% and still ends up with a quite wet crumb, so perhaps that would help you getting closer to your desired target - you were perhaps a bit overzealous when amping up the hydration?
The dough is quite soft and sticky (I don’t think classic kneading would work too well here), and is left to rise in a pan until almost fully proofed, then baked with a standard falling heat from 250°C to 250°C. The high hydration means it needs a significantly longer time to fully bake, aim for over an hour (65-75 minutes) and a core temperature of 98°C.
If you look at the photos, you will notice that the bread doesn’t have the light structure of a wheat bred, but is indeed still rather wet.
(1) Not going into the finer details of food chemistry and all that.