Oxygen will be allowed to naturally be deliverd to your plants as long as the soil/substrate isn't too compacted. A lot of growers like to use perlite/vermiculite to break up any heavy soils (usually clay) A lot of commerical bought soil bags will have a mixture of perlite in them or just add your own-it's cheap. Pro-Mix BX or HP is a good example and descent stuff for the price.
Hermie's are generally produced when the plant is stressed. It can be stressed resulting from lighting problems, cold, extreme heat, high humidity and general mishandling. I hope this helps some. good luck!