See: from the sublime to the ridiculous

• then and there
• the nature of the beast
• the new kid on the block
• the next rung on the ladder
• the nightmare scenario
• the nitty-gritty
• the nouveau riche
• the nuts and bolts
• the odd man out
• the odd one out
• the Old Bill
• the old-boy network
• the old country
• the oldest profession
• the oldest profession in the world
• the oldest trick in the book
• the old guard
• the old school tie
• the only game in town
• the operative word
• the order of the day
• the other day
• the other side of the coin
• the other side of the tracks
• the pants off