| Name | When |
|---|---|
| National Oatcake Day (UK) | Aug 8 |
| National Oatloaf Day | Sep 1 |
| National Oatmeal Cookie Day | 2X |
| National Oatmeal Day | Oct 29 |
| Oatmeal Monday (Scotland) | Feb 2nd Mon |
| National Oatmeal Month | Jan |
| National Oatmeal Muffin Day | Dec 19 |
| National Oatmeal Nut Waffle Day | Mar 11 |
| Ohio Wine Month | Jun |
| Okonomiyaki Day (Japan) | Oct 10 |
| World Olive Day | Nov 26 |
| National Olive Day | Jun 1 |
| Olmeca National Tequila Day (South Africa) | Dec 15 |
| One Cup of Tea Day (Japan) | Nov 23 |
| National Onion Day | Jun 27 |
| National Onion Rings Day | Jun 22 |
| National Opal Apples Day | Dec 9 |
| Open That Bottle Night | Feb Last Sat |
| Orange Chicken Day | Jul 15 |
| National Orange Juice Day | May 4 |
| National Orange Wine Day | Oct 6 |
| National Oranges and Lemons Day | Mar 31 |
| Oranges and Lemons Day (UK) | Mar 3rd Thu |
| Oregon Wine Month | May |
| National Oreo Day | Mar 6 |
| International Organic Day | Sep 22 |
| National Organic Harvest Month | Sep |
| Organic September | Sep |
| Orval Day | Mar |
| Oschophoria | Oct 2 |
| National Oyster Day | 2X |
| National Oysters Rockefeller Day | Jan 10 |
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