There are three holes in this oar. The hole that does not show up in this scan is in the handle. I have never read any mention as to what use the Vikings had for these holes.
The Viking Navy on the other hand has put these hole to good use:
The hole near the tip of the oar is used to put bending tension on the oar while the oar is being used like a water ski outrigger while sailing. This tension keeps the oar from vibrating. Also this tension is used to fine tune the position of the oar blade while under sail. The position of this hole so close to the tip has caused me to think that this oar has been worn down with use. Probable snow sailing. Thus the hole would have been maybe three times as far from the tip as it is now.
The second hole is near the shoulder of the oar blade. The Viking Navy design uses this hole as the tie point for the oar sails. The tension on this hole is directly back along the length of the oar so the oar sail tie tension doesn't interfere with the fine tuning tension of the first hole.
The third hole is near the fat part of the handle. The Viking Navy design uses this hole to tie the oar thru the hole in the center of the shield. The shield in this position is the brace that holds the oar out in the outrigger position.