As imafan said, there are lots of reasons that grass doesn't grow under any one particular tree.
That sap may be sticky but is not necessarily acidic. Oak leaves can be a bit acidic though, but the acidity is in the form of organic acids (vinegar is an organic acid, for example), rather than mineral ones, and those will decompose through bacterial action. Composting tends to bring any ingredients toward a neutral product.
Oak leaves can be a bit waxy and slower to decompose, so shredding with a leaf sucker or mower will make them go faster. I've composted metric tons of oak leaves and the resulting compost is just fine.