Neuroterus quercusbaccarum

Small (3mm), dark wasps. Asexual females may be abundant during spring emergence. 

If reared directly from the gall (please include photo of gall) be aware that it may be a parasitoid or inquiline - always check.  It must be keyed out and detailed photos included. 

Cynipoidea have complex life-cycles, and that of N. quercusbaccarum is a two-act play known as heterogony - an alternation between a sexual generation and an asexual (agamic) generation. 
The Sexual Generation (Spring):
The sexual phase begins in the spring, from January to March or April, asexual females emerge from the leaf litter where they spent the winter. These females lay their eggs in developing Oak buds. This triggers the tree to grow a Currant Gall. Inside these galls, larvae develop rapidly. In June, both males and females emerge. They mate, and the fertilized females then seek out the undersides of maturing oak leaves to lay their eggs. 
The Asexual Generation (Summer to Winter):
The eggs laid on the underside of Oak leaves cause the tree to form Common Spangle Galls. In autumn, before the leaves drop, the spangle galls detach from the leaf and fall to the ground. Inside the gall in the leaf litter, the larva develops into a pupa. Crucially, these larvae develop into females only. No mating is required for the next stage - parthenogenesis. These asexual females emerge the following spring. 
This alternating strategy is an evolutionary masterstroke. The sexual generation provides genetic diversity, while the asexual generation allows for a rapid population boom without the "resource cost" of finding a mate during the harsh transition from winter to spring.

Common and widespread but not often recorded except as the gall rather than the adult wasp.

Common and widespread but not often recorded except as the gall rather than the adult wasp.