How to talk about complexity without hand-waving
"It's O of n" is an answer. "It's O of n because I touch each element once and every map operation is constant" is the answer they wanted.
Two numbers, always
Give time and space together, unprompted. Leaving space out reads as not having thought about it.
Name what dominates
Complexity comes from one place in most solutions: the sort, the nested loop, the recursion's branching. Say which. "It's n log n, and the log n is the sort — the scan after it is linear, so the sort dominates."
Two variables, two letters
If the input is a grid, a graph, or two collections, do not collapse everything into n. Say "O of rows times columns" or "O of V plus E". Using one letter for two different sizes is the most common way to be quietly wrong.
Recursion: the stack is space
A recursive tree walk is O of n time and O of h space, where h is the height — and h is n in the worst case, on a degenerate tree. Say that. Forgetting the call stack is the most common space-complexity miss.
Amortised is worth a word
Appending to a dynamic array is amortised constant, not constant. Saying "amortised" costs you one word and shows you know the difference.
Say it out loud, in words
Complexity said aloud has its own vocabulary: "oh of n log n", "oh of one", "oh of n squared", "oh of v plus e". Practise it — the first time you say it should not be in the interview.