- The Meanings and History For Popular Idioms and Phrases
Thinking alike or understanding something in a similar way with others.

Example: My brother is not 'on the same page' as me when it comes to music.
The origins of this phrase are unclear, however, one might guess it stems from schools, where teachers and students must be on identical pages in their books. Others have their own ideas as to its origins, but it doesn't seem like the origin of this expression is really known for sure.

As for this saying's age, it appears to be a recent one, as I could only find it in writing as far back as the 1970s. In the year 1974, for example, the expression is used in the Corona Daily Independent newspaper on the topic of football: 

    "I think we can beat Washington and whichever team we play next to get into the Super Bowl. If 47 players 
     and our coaches are all on the same page, we can do it."
* The rules will be changing around the office this upcoming Monday, so I wanted to be sure and tell everyone far enough in advance so that we can all be on the same page once they hit.
Note: The origins for most popular idioms are nearly impossible to find, if not completely! What's provided are the theories that may be plausible to how a phrase originated, but not necessarily so. 

In addition, quotes that contain a phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means shows they originated from these. In all likelihood, if an expression is being used in a newspaper, it's probably already well known, and thus, from an older period of time.
O-Letter Sayings  |  Know Your Phrase - Common Idioms and Phrases, Their Meanings and Origins

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Phrases and Idioms Starting with the Letter - O.
Phrases and Idioms