Well, personally, I think an OOO is a much cooler guitar than a D, but that is a personal bias. Most people do not seem to agree with me. Change the numbers I said earlier to $3000 for excellent condition, and $2000 for average.
Just a quick lesson in Martin model numbers. Bear in mind that there are many exceptions. The part of the model number before the hyphen denotes the size and shape of the body. The part of the number after the hyphen denotes the style, which includes the type of wood used on the back and sides, the grading of the wood, and the decorative elements (purfling vs. pearl, as well as how extensive the pearl is). So, for instance, a D-45 is a Dreadnaught with a two piece rosewood back and side, best quality materials, ebony bridge and fingerboard, cream binding, pearl on the fingerboard and peghead, and pearl purfling on the top back and sides.
A O-28 is a parlor guitar with two piece rosewood back and sides, ebony bridge and fingerboard, black and white purfling (if it is old enough, it would be herringbone), and a decal on the peghead.
A OOO-18 is what I would call a concert sized guitar with a mahogany back and sides, a rosewood bridge and fingerboard, black binding, simple black and white purfling, and a decal on the peghead.
A C after the style number denotes a cutaway (a recent development), an S after the hyphen means it is a twelve fret guitar and has a slot peghead. An H before a D means it is has herringbone purfling and forward shifted scalloped bracing.
At any rate, I personally love mahogany guitars, and I love midsize and small guitars. To my mind, that is one hell of a cool guitar you've got there. Enjoy it.
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