Consider the following quotes (emphasis mine).
For a split second, I meet eyes with an older man standing in a still gaze just opposite of me amidst the sudden chaos. source
Taking a seat on a stool just opposite of me, he gives me a huge smile. source
Guards stood at my door and one sat just opposite of me, his eyes never leaving my face. source (see page 39)
I was surprised when I caught sight of Alex, who was waiting just opposite of the bathrooms. source
They were standing just opposite of the road and looked hungry, so we picked some of the grass that was just out of their reach and feed them. source
There were two groups of people standing just opposite of each other. source
These quotes are all using opposite of to mean "across from" or "opposite". Are these uses acceptable in formal English writing? Or do they imply a less formal register, i.e. are these meant to merely be reproductions of casual speech?
Please also consider this NGram: source