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