For more than a half century, the Oakland-based artist and retired art professor Oliver Lee Jackson has been steadily creating his arresting paintings, sculpture, mixed media, drawings and prints. Now 86, Jackson seems to be at the height of his artistic powers, masterfully blending materials and mediums. The newest works in his solo exhibition at di Rosa Center for Contemporary Art in Napa are sculptures made during the nearly past two years of the ongoing COVID-19 pandemic.