Why do some oak trees retain most of their leaves until spring?

Answer 1

Because they can.

The advantage of holding onto dead leaves overwinter is that they will add to the oak's mulch (not another tree's) in the Spring. Thus, the oaks can recoup some of their loss.

The danger of keeping dead leaves on a tree is the formation of icicles that would snap branches of the tree. Oaks are incredibly strong, and can survive the formation of such icicles.

Interestingly, beeches also keep their leaves but only on the lower, stronger branches.

Answer 2

In California we have several "live oak" species that never drop all of their leaves at once. Other species lose all of their leaves seasonally.

Basically, different oaks have adapted to conditions in different areas with varying climate and soils. Keeping their leaves -- or dropping them -- is away of maximizing available resources.

Answer 3

It is thought that beeches (and perhaps oaks) keep their leaves to shade out competitors.