Oricon is a pretty good rough guide of sales, but it depends on the type of artist. Singers like Nishino Kana are massively popular, but her sales don't seem to be, looking at Oricon charts, since a lot of her sales are through mobile downloads. iTunes and other internet download services aren't counted as well, and some stores may not be counted.

In general, you can use Oricon as mostly a guide to judge relative popularity, keeping in mind those factors.