Again, you're absolutely right wrt. proper, formal usage. But big-O is also a part of programmer slang now. The OP used it in this form.

(It's also faster to write, or even say out loud, "O(n)", than it is to write/say "proportional to number of").

Frequently when writing O(n), the writer is making a comparison. e.g. "The GC was O(n log n) wrt memory, but now it's just O(log n)." That expands to a lot more text.

Or, to put that another way: there is a constant cost in human parsing complexity when writing "O(n)", but the number of parse-nodes in the text when using "O(n)" only increases as O(n), whereas without "O(n)", it increases with O(n^1.3). ;)