For a town of fewer than a thousand residents, Orta has an unusual concentration of good restaurants — partly because of its day-trip popularity, partly because of Villa Crespi's gravitational pull on regional chefs. The lakefront restaurants on Piazza Motta and along the waterfront promenade are the obvious choice for lunch, with terraces facing the island; quality is solid rather than exceptional, and prices reflect the view.
Locanda di Orta, a small place in the historic centre with one Michelin star, is the best mid-range option for dinner: tasting menus rooted in Piedmontese cooking — agnolotti, vitello tonnato, Bra sausage — at a fraction of Villa Crespi's prices. Venus on the waterfront does dependable lake fish: perch (persico) on risotto is the local signature. For something more casual, Edera does wood-fired pizza, and the small bakery off the piazza serves espresso and lakefront-priced pastries from early morning.
Note that many places close in winter (roughly November through March), and the few that stay open keep shorter hours.