Burnt Umber and Burnt Sienna over a very light buff Tamiya acrylic base. You can add transparent colour filters (Tamiya Clear Yellow etc.) over the top if you want to, but I generally like the plain oils.
Also, once you get the hang of it, you can have fun trying additional colors for both the oils and base. Along with the colors The Red Baron mentions, Raw Umber, Raw Sienna, and Van Dyke Brown, are ones I've used. He mentioned using water-based oils but you can also use conventional oils and by adding a touch of naptha (cigarette lighter fluid) or other drying additive to the paint it speeds up the drying. For the base, you just need to make sure you use something that won't be affected by the oil paints or thinners. Along with Tamiya paints I've used several of the various AMMO Mig or Vallejo acrylic wood tones and even some of the new urethane primers (Badger Stynylrez or AMMO Mig One Shot) which are available in various colors. This is the prop for the WNW Roland C.II I'm currently building and was done with Burnt and Raw Sienna over brown and pale yellow Stynylrez and AMMO primers.
The cockpit floors are Burnt and Raw Umber over Vallejo and AMMO acrylics. The leather seat was also done with oils over black Stynylrez primer.
Please let me know if you'd like any more information. Doing wood effect with oils is great fun one of my favorite parts of building WWI aircraft.
Cheers,
Eric