What kind of tree is it? I'm not familiar with "Oregon Pine".
We work with many grafted trees in bonsai. Not a good idea to bury below the graft, we like to see the nebari, or surface roots. Nebari is that area of the tree where the tree emerges from the soil, where the trunk joins the roots. The lower trunk and nebari are the most important design element of the bonsai.
Now, for the graft: the best grafts are low, close to the soil, and have healed well so that there is not a bulge at the graft union. Different species grow at different rates, so often the rootstock grows fatter faster than the scion part. Often the bark of the two species are radically different. Hopefully you have a nice one. It looks quite healthy.