I want badly to take up all the carpet in my house and install hardwood floors.

However, my house already has a great deal of oak - doors, window frames, floor boards.

How do I "blend" new wood with the old? Should I think of a darker shade? Should I worry about the grains?

It's a matter of taste IMO. My home is all different woods, I like that. I have kept the wood tones on the medium side. Personally I'm not a big fan of seeing light and dark woods together.

Designers mix woods all of the time.
You could always look online at room scenes to get some ideas.
Seems like a fun way to add unique character to your home.

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