I believe you are on the right track. The oxygen atom, will cause the electron density around the C-H bonds to decrease or be withdrawn, because oxygen is quite electronegative. Consequently, the protons will be less shielded. When deshielding occurs, the signal will be more downfield.
Shielding produces an upfield signal.
It might help if you take another look at Figure 13.17 on page 555. Hope this helps.