"...I have a fireplace..."

Most fireplaces are very inefficient, wasting a large percentage of the heat generated by the burning wood. An insert would make a huge difference, but only if you can have a supply of wood on hand.

A propane powered heater such as the Mr Heater will help, and they have a low oxygen level sensor that will turn them off if necessary.

A propane powered catalytic heater might do the trick, but indoors I would only use the one pound throw-away canisters. If you go for a refillable bottle (like those used on a BBQ), leave it outdoors and run a long hose in. And I would turn it off before going to sleep.

For a small area, you can take a one pound coffee can (a metal one of course), stuff a roll of toilet paper (minus the core) into it, then pour a bottle of rubbing alcohol in. Light it and you get heat. When the TP starts to turn brown, snuff it out, let it cool, then add more alcohol and relight. Be advise that you can't see the alcohol burning in daylight, and for sure don't knock it over!!!