adding water to heating system
Disclaimer: what I know for sure and $5 might get you a good cup of coffee somewhere.
When you look at your boiler you see a lot of pipes. You might want to take a nice hot cup of coffee or tea and a flashlight or trouble light so you can completely see all heat system around your boiler, and take a long hard look at what's down there. You really want to look at pipes and know what they are for and what is carried in them.
Aside from the line or piping that will carry your fuel oil to your boiler and burners (locate and identify this one first), realistically the only thing in your pipes will be warmer water leaving your boiler most likely through a top pipe, cooler water returning to your boiler usually with the pump (circulator) pumping the water into the boiler's side, and a fresh water line with a valve to completely turn off fresh water to the boiler water loop (this valve is so if replacing boiler techs don't have to turn off entire house's water supply).
I've got to get to bed. If this were my house, I'd locate the system pressure relief valve. It's in the piping loop near the boiler. When the system for whatever reason gets to much water pressure rather than explode your boiler it releases excess water onto your floor or preferably down a drain that's directly under your relief valve.
When you open your fresh water valve, your system should begin refillilling; however because you have so much air in the system, your relief valve will begin discharging water because the excess air has no where to go. You say you leave the upstairs open and the water never comes out? When you open the fresh water supply that upstairs air should come out. Most systems have a valve that automatically refills your system but some aren't set up that way.
If it were my house I 'd have my wife upstairs with radiator key asmall plascic containeer under the radiator bleed key and her on her cell phone talking to me. I'd ask hher to open the radiator bleed valve. teell me if you hhear aiir coming out, then I'd openthe fresh water intake valve. And I woould want to hear her say that air is coming out. As soon as she says water is cominng out and and she's turninng off raditor bleed valve,I'd turn off fresh water valve. II'd bleed the other house radiators. If the boiller relif pressure valve trips off while i'm fillinng system. I' d turn off , bleed system at each radiator, and nort open fresh water valve so far while topping off system. Good luick and good night Cliff