HDO is short for Hors d'Oeuvres, which are passed "butler style" by waiters at events. I love servers who offer to clean the platter (using water and vinegar) when they return to the kitchen. That's much better than handing it back all smeared and messy. I notice and appreciate the help; sample nibbles for you, good server!
Here's a recent sampling of HDOs served at a Bay Area event:
Endive & asparagus spears with dijon sauce
Butter lettuce with shrimp, toasted coconut, shallots, ginger, lime, peanuts, and a smidgen of jalapeno
Artichoke cake with sharp cheddar and onions, baked