Ask HN: Why is big O often used in CS when omega or theta should be used?

I've found some people on a well known QA site to be so strict with big-O notation that I don't use it anymore and instead only refer to constant, linear, quadratic time and so on. Even when O(1), O(n), O(n^2) would be actually easier to write even if they are technically wrong.

Finding lower bounds is generally more difficult.

Those aren't in the ASCII table.