"Oo-er" is a term apparently started by the popular British series "The Confessions of Georgia Nicholson". In the books, the phrase is used whenever someone mentions something scandalous or neat. It can also be used whenever something suggestive has been said, intentionally or unintentionally.
Rosie: Actually, Sven and i have gotten to number 8 (upper-body fondling in bed).
A word to express one's interest in something. Normally used by girls when subtly stalking hotties on the street; or when they're shopping and find something exceptionally cute or sexy, such as clothing.
Friend #1: "Check out that hottie across the street!"
Friend #2: "Ooer!"
Friend #1: "Look at this adorable Guess wallet I just found!"
Friend #2: "Ooer, very sexy."