AON differs from fill-or-kill in timing. FOK demands the whole size immediately or cancels; AON is patient and simply refuses to be broken into pieces.
Because an AON order cannot be partially matched, many venues do not display it and do not give it normal price priority. It can sit unfilled while smaller orders at the same price trade around it.
Example: you place an AON buy for 10,000 shares at 8.00. Sellers hit 8.00 in clips of 2,000 all afternoon, totalling 30,000 shares, and you receive nothing, because no single counterparty and no permitted aggregation satisfied your 10,000 at once. A plain limit would have been done in minutes.
Related: minimum-quantity, fill-or-kill, immediate-or-cancel