The hole is most likely either the peg hole for the toplever spring (near enough correct position) or a pin hole for a retainer/guide pin from the safety pushrod which is often pinned to the underside of the top strap.
Usually these are blind holes but I have seen several guns over the years with through holes and the pin sitting flush with the external surface of the metal.
It is not an oil hole! Whatever you do, do not squirt oil into any hole or crevice in your British shotgun, it will ruin the wood.
If it is either of the holes as I describe above, it shows that the original pin has backed out of its hole which is not a good thing but maybe not catastrophic. Only a strip down will show.
