I have dug up some old house posts out of the garden.

Rot and ants have hollowed them out and taken chunks out, but they are still fairly sound.

I gave them a good wash and scrub and they look quite nice.

I had a mind to turn them into lamp posts and install down my driveway.

What is my best option for preserving them?

Just oil?

Just stain?

Stain and gloss?

Set them in concrete and fix a small tube at the bottom where you can regularly pour some wood preservative in to keep the bottom of the post from rotting. A good wood stain varnish will look after the top sections but has to be done every year at least

Excuse scribbled drawing

Just oil?

Just stain?

Stain and gloss?

Oil is a good start, but you might need to repeat this over the years. If the logs are hollowed out you need to treat the inside too. I think, some sort of "top cover" might be useful if used as a post.

What kind of timber is this? If you do you know this, it would help you to understand the required future maintenance work in terms of regularly treating it.

Stain and gloss, will provide some protection, but it's also a matter of taste. If the surface of the wood is nicely aged, I wouldn't stain it. Gloss, in my opinion, is more something for a boat or yacht, not really for a lamp post.