How to say Big-O notation out loud in a coding interview
3 min read · Updated 21 October 2026
In almost every coding round you will be asked “What's the time complexity?” Saying it clearly sounds confident. Stumbling over the notation can make a correct answer sound unsure.
How to say each one
| Notation | Say it as | Typical example |
|---|---|---|
| O(1) | “O of one” | Hash map lookup (on average) |
| O(log n) | “O of log n” | Binary search |
| O(n) | “O of n” | One pass over a list |
| O(n log n) | “O of n log n” | Efficient sorting, like merge sort |
| O(n²) | “O of n squared” | Two nested loops over the same list |
| O(2ⁿ) | “O of two to the n” | Trying every subset |
| O(n!) | “O of n factorial” | Trying every ordering |
Explain it, don't just name it
Give the complexity and the reason in one sentence:
- “Time is O of n, because we go through the array once.”
- “Space is O of n, for the hash map of numbers we've already seen.”
- “Sorting first costs O of n log n, and the scan after it is only O of n, so overall it's O of n log n.”
Mention both time and space
Interviewers usually want both. If there is a trade-off, say it out loud: “We could do this with O of one extra space, but it would take O of n squared time.” Naming the trade-off shows you understand the problem, not just one solution.
Common slips
- Saying “O n”. Add the “of”.
- Forgetting the cost of sorting when your solution sorts first.
- Saying “log n” without pointing to the part of the code that halves the work.
- Calling a hash map lookup O of n. It is O of one on average.