Q:

Is this an oxymoron, and what would be the purpose of making seemingly illogical statements in writing?

Here's a piece of writing I came up with:

The rolling billows rocked the mighty galleon
  cradling it madly as if it were but a mere child.

There are many seeming contradictions: When we think "billows rocked" we think of a powerful motion, "cradling" goes against that, and then "madly" goes against cradling and finally "as if it were but a mere child" goes completely against the idea of a powerful "rolling billow"? 
Is this just really bad style?  It seems to have a poetic effect.  Is there a way to leverage such inconsistencies to deliberately create this effect?  And can it be used in novels, or only in poetry?

A:

Poetry forces us to see things in a different way, so it often uses language in ways that we wouldn't typically see in prose.  Grammar that is technically incorrect, words used in non-standard ways, deliberate misspellings, idiosyncratic punctuation, and odd juxtapositions are just a few of the "disruptions" occasionally practiced by poets.
In this case, the deliberate mixing of contrasting metaphors is actually fairly effective in conjuring up a vivid mental image --the mind is forced to work just a little bit harder to find a way for it all to make sense.
It's quite common to occasionally use poetic techniques in prose writing.  But they are perhaps best used sparingly, since they can interfere with the literal transmission of information that is the central focus of prose.

A:

I don't think it's an oxymoron. Sure thing, you have chosen a strange mix of images to evoke - mainly due to the contrast between "cradling" and "rocked". 
But as far as I read it, it's a legitimate metaphor: the ship in this case is not bigger than a child in a cradle compared to the storm outside. The juxtaposion of the single elements may seem oxymoronic at first, but at large the "ship as a cradle" idea is not unheard of. It's not even so illogical, it just requires a little stretch of imagination. 

Is this just really bad style? It seems to have a poetic effect. Is there a way to leverage such inconsistencies to deliberately create this effect? And can it be used in novels, or only in poetry?

I don't think it is, but then again it's opinions. You are already leveraging the effect of the metaphor and contrasting images. And yes, it could.  

