Is Greek oregano really different from "Mediterranean" oregano?

Yes, and it starts with a different plant. Greek oregano is Origanum vulgare hirtum, a sub-species that grows wild on the slopes of Mount Olympus. The "Mediterranean herbs" oregano on most shelves is a milder, broader mix.

What does altitude actually do?

At 1,000 metres and above, the growing season is shorter, the nights colder, the sun harder. The plant responds by concentrating the essential oils that carry its aroma. That is why a pinch of high-altitude Greek oregano carries a dish where a spoonful of bulk-grade oregano barely registers.

How can I tell good oregano from tired oregano?

Two signals together: a vivid green colour, preserved chlorophyll from hand-harvesting and low-temperature drying, and a strong, immediate aroma when you crush a little between your fingers. Brown and faint usually means over-dried or long-stored.

How do Greek cooks use it?

Over tomatoes and feta, on grilled meat and fish, through tomato sauces, and steeped as a simple herbal tea. Organic Greek Oregano, Mt. Olympus.