Hi @eloise89,
Thank you for your question.
What you are seeing is not damage, it is a knot in the timber. Knots are locations where branches extend out from the main trunk of the tree. The change in direction of the tree fibres will create a point of slight weakness in the milled timber which can cause minor cracking like you are seeing.
Unfortunately, there is not a great deal you can do unless you are willing to replace the decking board altogether.
My suggestion is to lightly sand it with 120 Grit Sandpaper to minimise the appearance of the cracks, then clean off the sawdust and oil as usual.
Check out How To Oil Your Deck for some guidance.
Let me know if you have any further questions.
Jacob
