This is a widespread and sometimes very common species, though generally local. In Switzerland it flies in rather hot places in valleys where there are plenty of nectar plants and scattered small oaks. In such places it may often be seen in good numbers in June and July. Further south it flies higher in the mountains, being common along tracks in the Pyrenees up to at least 1600m and doubtless higher in places.

Superficially, the ilex hairstreak resembles the clumsily named false ilex hairstreak (clumsy and inaccurate, as there is nothing false or deceptive about that butterfly!). It is, however, a very different beast. The overall impression is of a darker, sleeker butterfly with a strong, though broken hairstreak. On the hindwing this hairstreak is bordered internally with black and the sections are usually folded into rather deep 'v' shapes in the vicinity of the anal angle - or at least follow a strong curve there. In false ilex hairstreak they are flat or slightly curved in this region. Another useful feature is the sequence of orange spots on the hindwing, which have no real hint of red and decrease in size as they progress away from the anal angle. In false ilex there is often a distinct red tinge to these spots and they are more or less the same size. After a few of each species have been seen there is little chance of confusion except in the case of very worn individuals.

Eggs are laid on oak (I'm not at all sure where the species gets its common name from), usually on the trunks or low branches of small bushes, some 50cm or so off the ground, from where the emergent caterpillars can crawl to leaf buds in spring. The only eggs I have found have been parasitised. The butterflies themselves emerge in late May or early June - or perhaps later where they fly at altitude. I have found abundant, fresh butterflies in the Pyrenees in late July.