What happens if you put antifreeze in where the oil go?

If you start and warm up your engine:

1) The oil gets very foamy and mucky, and does not circulate through the engine properly.

2) The water in the antifreeze can heat up until it turns to steam, which expands with great force, and breaks some part of the engine.

It's best to have the oil changed. That will empty the engine of the oil and antifreeze mix.