The problem with "an hotel" is that is assumes that people use the RP pronnunciation of 'otel, as lots of people these days say hotel with a "hu" it doesn't work so well.

I've noticed it is often still written an hotel but pronounced uh hotel with a "h", just to complicate things even further.

I don't know why the trains say oh nine hundred hours, must be a military throwback. I'm sure someone will know.

I would never say nine o'clock am - the "o'clock" is redundant - its nine am, surely???