I scheduled five classes today. My last class will finish at 7 pm. I will listen to a talk about a French guide. He is a guide who induced a French museum. I watched his YouTube video/channel. It was very exciting, so I'm looking forward to hearing to his talk. Then, I want to hear why he became a guide in French(France).
In written English, we seldom use "o' clock." It is usually only used in conversation (spoken English). We also seldom use the 24-hour system (06:15, 19:00). It is more common to use the 12-hour system (6:15 am, 7 pm or 7:00 pm).
It is common to see the 24-hour system used during travel (flight or train schedules). Example : His flight will depart at 21:30 and arrive at 00:45 the next morning.