can you use orianna legend only once per turn

I can't confirm — Orianna doesn't appear in my card data at all (no Legend card or champion tag under that name), so I can't read the actual ability text and can't rule on it specifically. If you can tell me the exact wording of the legend ability, I'll rule on it.

The general framework, though: if the ability is written as a cost followed by a colon (an Activated Ability), it can only be used on your own turn during an Open State, and if the cost is exhausting the legend, it stays exhausted after one use — so once per turn unless something readies her, per Rule 381. If instead it's written "When…" or "At…" (a Triggered Ability), then it isn't limited to once per turn at all; it fires every time its condition is met (FAQ #3291 pattern).

Which category it falls in depends entirely on the printed wording, so send me the text and I'll give you the exact answer.