You could definitely get a Rosemary strap in oak darwin - but it's not long enough to be worn messenger style.
I don't think the messenger straps come in leather if that's what you're asking. I mean, some of the messenger bags *do* have leather straps, but they don't seem to fasten onto a D-ring iyswim.
If you check out the messenger bags on the website, you'll see that the oak bags come with brown straps nowadays, although previously they came with cream. I don't know if they do a canvas strap in oak colour.