Orcs (orcneas) have been mentioned in classics like Beowolf and are as termed in Old English dialect, evil infernal deities from the Underworld. I love that. The word, Orc, has also been spelled as orke and even orge and centuries ago orges were referenced as giant beasts and were usually written about in fairy tales. Every one is quite familiar with orges, but orcs as Tolkien created, are essentially goblin monsters.
"Orc is not an English word. It occurs in one or two places [in The Hobbit] but is usually translated goblin (or hobgoblin for the larger kinds)."
The most common orc we think of today, it Tolkien's. He has two that I'd like to point out. One is a weaker, smaller one called the Orcs snaga--which means orc slave according to Wikipedia. The other which I believe fits the classic monster realm is the Uruk-hai orcs. These beasts that Tolkein created and that Peter Jackson brought to life are an amazing breed. They are oversized, menacing goblins who eat man-flesh. Yikes! Remember that? If I saw one of these creatures, I would run. FAST! These beasts were created or unearthed rather from Saruman, the White Wizard. From there many fans and fantasy historians speculate about the Uruk-hai orcs origins. I found some discussions on Saruman's cross-breeding with men, and then another mention of them cross-breeding elves. Honestly, I couldn't find anything definitive. The bottom line is their scary-cool. I'm not sure if you think Orcs are a classic monsters, but I do.
Did you have another idea for the letter O? A different Ogre, Oscar the Grouch, perhaps?