I was reading a book that had a sentence containing this:
...onetime commissioner of New York...
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The other meaning is "of or relating to a single occasion: a one-time charge."
So it can be both. The sentence you provide has not enough context to decide which one of those two is the right meaning for your case.
I've been to that Stadium only once.
Same goes for "two times" which is "twice". These are the only exceptions, after them you say: Three/four/five/etc... times.
(see comments too)
Strangely, my American Heritage dictionary says that onetime (not hyphenated) means former, and one-time (hyphenated) means only once. I'm not sure I completely believe this distinction, at least in terms of how it is actually used. I suspect many people are completely unaware of this distinction (as I was until I looked it up this morning).
Certainly, when it is used as an adjective (either meaning), it should be spelled onetime or one-time. Otherwise it should be two words.
One time only! A special concert for ...
One-time pads are the only truly secure cryptosystems.
John, a onetime professional basketball player ...