| 1 | on loan |
| 2 | on one's own |
| 3 | on one's way to |
| 4 | on purpose |
| 5 | on sale |
| 6 | on short notice |
| 7 | on the dot |
| 8 | on the right track |
| 9 | on the side |
| 10 | on the tip of one's tongue |
| 1 | on the whole |
| 2 | on time |
| 3 | on top of |
| 4 | once and for all |
| 5 | once every other… |
| 6 | once in a while |
| 7 | once more |
| 8 | one day is as good as the next |
| 9 | one in a million |
| 10 | one step at a time |
| 1 | open a savings account |
| 2 | or so |
| 3 | or something |
| 4 | oral presentation |
| 5 | order sth. From a catalog |
| 6 | order through mail |
| 7 | organic chemistry |
| 8 | other than |
| 9 | out of bounds |
| 10 | out of breath |
