oak post below ground

Sitting on metal screws????? More information would help!

 

I hope the metal screws are stainless steel or brass. Oak will sort the steel out double quick if they're mild steel/semi-iron.

Your builder is quite right, oak has amazing properties in all sorts of applications. Think of Tudor/Jacobean buildings. So yes, your oak posts are quite OK below ground. Although what section dimensions are the posts?

 

Tis strange that the builder has bothered with the steel and then set post below ground level, it defeats the object of the steel which is normally used to raise the posts above ground level.

 

Tony, can you clarify "...the posts sitting on metal screws but they are below ground level,..."?

Do you mean the post ends are below ground level or just the SS pins?

If the former, then - as Phil says - that's weird. How far does the actual oak post go below ground level? If it's, say, a couple of inches, then you could consider digging a round or square recess a few inches deep and a few inches around the post bottom to expose the SS pin and bottom of the post and putting a layer of decorative chippings down. That'll keep the oak post above 'ground' level and could also be a feature.