Aside from a worn out O-ring on a Corny keg lid, are there other factors that may contribute to very, very faint leaking? Like maybe not enough pressure is being created by locking the handle, etc.? I have swapped around O-rings that I know were making a seal on other kegs and still get a real tiny hiss around the edge. Any thoughts or suggestions?

I guess I try a brand new O-ring but am afraid to open up and change (oxidation) or is that a non concern? Kegging newbie needs some schooling.

Mostly, it is just age.

Some sealing tricks of mine:

I lift the keg by the lid handle and sort of rock it a little while adding head pressure. This usually seats the keg lid, then I can lock down the handle.

Sometimes, I have to leave the lid leaver up a little to more firmly seal the lid.

The lid seal can often be restored to round by heating it in hot water for about 30 minutes. These seal best when dry, though.

Bits of dirt and old rubber get into the areas where seals meet. This includes the pressure relief valve and the poppet on the connectors. A toothbrush or small brush and PBW will usually clean this up.

When empty, I clean and sanitize my kegs and store them under pressure. This usually makes reusing them a snap.

Not sure what pressure you're at, but I find that I sometimes have to nail my kegs at a higher pressure (30psi) for a moment to create the seal, then I can back it down and stay sealed. I'm not sure why this is the case, but it happens to me!