What is the mass of carbon dioxide absorbed by an oak tree in one year?

Re: Bryan Krause's comment, measurements per hectare are common. However, if you can just estimate average trees per hectare, then it works out. But it still likely depends on the local climate, ecology etc. It also varies across the life term of the tree.

Some estimates have been made, such as in this paper (see Table 4), for Quercus rubra and 2 other hardwood species. They estimate that for the most mature trees, Q. rubra stores ~20 Mg/hectare total biomass, of which approximately half is carbon.

This is a little more than twice the biomass of the 8-year-old tree, so if we just linearly interpolate we could say that a hectare of oak stores ~1 Mg C per year. There are some problems with this- for example, older trees are likely to store much more than that, but it is not clear exactly how much variation there is over the course of an oak's life, and also there is likely variation between the sites in the study.

It is also clear that the other species measured, e.g. American chestnut, are more effective carbon sinks than oaks as young trees.

See the paper or this pop science article on the subject for more information. THere are also a number of papers citing that paper that may be interesting, such as this, this, and this.