It sounds like you already have many/most of the skills needed Ongie! I believe that most/all of the skills required need on-the-job experience to actually be prepared to 'manage'.
If you can bring/learn common sense, patience, great listening skills, communications, multi-tasking (most importantly), negotiating and perserverence those would be some of the top things which either you have those skills or are willing to learn them. Sometimes you that can only come from the school of experience.
People can have all of the computer/software/technical skills in the world however becoming an office manager truly takes much more than that.
You need to be able to work with all kinds of people such as vendors, customers, clients, employees, have negotiating skills, need to be able to think on your feet and hit the ground running, can change thoughts/tasks on a dime, be able to deal with any situation that is thrown at you, be prepared for the worst and the best, have the patience to work/train people and have extremely broad shoulders to hold up all of the hats you will be wearing :-)