do (your) homework

to learn everything you need to know before doing something.

If you had done your homework, you would have known it was a silly question to ask.

• oil the wheels
• Old Bill
• old-boy network
• old country
• oldest profession
• oldest profession in the world
• oldest trick in the book
• olde-worlde
• two-time
• two-timer
• two-time someone
• two-timing
• two-way street
• Uncle Sam
• under a cloud
• under a microscope
• under arrest
• under attack
• under belt
• under breath
• under certain circumstances
• under false pretences
• under fire
• under heel
• under lock and key