"Opposite of" is used to differentiate the essence or the main features of two things: "Bad is the opposite of good", "Big is the opposite of small". "Opposite to" is used to express that two things are physically located in opposite places or facing each other.

Example: The opposite of white is black. Example: White is the opposite of black. You use "the opposite of" to tell us what the opposite of something is. You us "opposite to" a little differently. Example: The effect was opposite to the effect they wanted. It's hard to explain the exact difference. When you use it this way, it's more like something that contradicts something else.

For posterity's sake, I wanted to add that Americans often use the expression opposite of more commonly than opposite to even with respect to directionality/physical position. British people are more apt to say, for example: The grocery is opposite to the coffee shop. While Americans might say: The grocery store is opposite of the coffee shop. OR, more commonly: The grocery store is across the street from the coffee shop. Opposite of is generally used when are refer to two concepts, like big is the opposite of small.

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