Yes and No, OWB mag holders can be concealed very easily with a cover garment. However there are other things involved and in some situations an IWB mag holder is a good option. For a man of your experiance I would think that you would know that differant options work better for differant people.
For instance I carry my firearm on the strong side and my mag holder on the weak side. I carry my wallet on the week side rear pocket so when I have to lift my shirt or vest to get my wallet I do not expose my gun. However it is easy or possible to show the mag holder in this situation. Is it a real problem if somebody sees it? No not for me but it might be to others. So I got an IWB mag holder for those times I really want to stay concealed. It is not at all uncomfortable if they are done right.
Try TT Gun Leather they do great work. He makes a very nice single and double IWB mag holder. He spreads the loops out to keep the bulk down, I have one and love it.
TT GunLeather High Quality Leather Gun Holsters