On horseback
On the back of a horse.We went to the campground on horseback.
On hold
To be left waiting on a telephone line.I phoned the bank but I was quickly put on hold.
On hold
To be waiting, to be temporarily halted.The construction of the building is on hold while the city engineers finish their inspection.
On hand
To be present.The speaker will be on hand after the lecture to answer questions.
On hand
Nearby, within reach."Please keep your dictionary on hand during the exam."
On hand
Available.I do not have any aspirin on hand at the moment.
On guard
Careful or wary.The store owner has been on guard since he was robbed last month.
On edge
Nervous or irritable.The boy is on edge because of his exams.
On easy street
Having enough money to live comfortably.My uncle has been on easy street since he sold his house and invested the money.
On duty
At work, currently doing one's work.There was nobody on duty when we arrived at the swimming pool.