Oak Tree

The rhythm for the poem isn't great. The lines start out much longer and then get much shorter as the poem continues. While a few short lines here and there may emphasis a point in this case it creates an irregular rhythm which doesnt compliment the rhyme scheme, even though that is irregular.

There was a giant oak tree,
in a green grass field.

Good first two lines, they aren't striking or anything like that but it gives a setting.

It's branches tall, its leaves so green,

A good, if little predictable description. I think that you should try and set these oak tree apart from other oak trees, so an inventive word for green would go down well here.

and strong bark was it's shield.
Squirrels darted here and there,
the acorns were everywhere.

All good, the rhymes working well, nothing I can really point out. I like the description of the strong bark as a shield.

untill one night,
the wind, it blew,
the branches flew.

Now here it gets shorter, maybe if the last lines were made into a new stanza where every line was shorter it would flow better. Also I think you need some sort of linking word in there on the last line "and" or something would do nicely.

And when the first rays of light touched the ground,
there in a mound,
was the tree.

Now here is a place where "was the tree", as a shorter line, is good at adding emphasis.

Overall the poem was nice, it didnt have any hidden or deeper meaning however. It was nice in its simplicity though.