One path to pretty is to pull it out and put something new in. Oleanders are mostly all dying due to a blight and yours looks like one of many lining my driveway, about a year or two before they die completely and husband pulls the stumps out with the pickup truck. In the short term, I wouldn't trim it too much and would deep soak it regularly and you may get it to bloom.

I'd pitch it, I had an oleander that looked similar and could not dig it up without a bulldozer as the roots were in so tight. I put ground clear on the roots. I removed all the other oleanders as well that were in the garden when we bought this house. The prior owner put those in with hawthorn plants that all looked a little ragged for wear. I put in purple hop seed, thatching reeds, and Mr. happy echium which can all handle dry weather and still look cool. The deer here are not interested in any of those plants.

High Desert--not much you can do right now except wait until autumn. Once you get to about mid-October you could cut it back by about 1/3 and then water it like crazy all winter. Deep soak--let the hose trickle at the base for 12-24 hours every couple of weeks. By spring it might fill out for you.

Whatever you decide, wait until October, because going into summer's ferocious heat, nothing new is going to survive, and trying to stimulate growth on an established plant that is going to go somewhat dormant to wait out the summer is a waste of water and effort.