What's the difference

1.on hand 在手边,在附近,在场
Ms.Sharp will be on hand to answer questions later about the film.
The two cosmonauts have seven hours of oxygen on hand,but the job should take only four hours.
2.at hand 在手边,在附近,即将来临
I don't happen to have your application at hand at the moment.
With the holiday season at hand,everyone is very excited.
3.close at hand 就在眼前
The town is close at hand.