Osnaburg is a natural-colored, bottomweight muslin-like fabric. It is highly recommended for beginners because of the low cost and excellent wrap qualities once broken in. It works great for wraps and ring slings and breaths nearly as well as linen. Because osnaburg doesn't come in a lot of varieties, it is almost always an excellent choice, and the bolt tags are always clearly labled (although sometimes hard to find.) The only thing to look out for is "J Thompson" brand osnaburg, which has a reputation for being much thinner than other brands and less desireable. Roc-lon is the brand most people use and love. Although "traditional" osnaburg is made from linen, most of the osnaburg available in stores is 100% cotton.
Because osnaburg is sold raw and unprocessed, it has a lot of "gunk" on it from the manufacturing process and it is recommended to scour the fabric before using it for your child. It is a fairly simple process but should not be skipped.
Outside of the US, osnaburg can be insanely hard to find and sometimes very expensive. A decent alternative is heavyweight calico, but make sure you shop by feel since calico varies by weight. You want something sturdy enough that you could make pants out of.
"Calico" outside of the US is a raw, sturdy, unfinished fabric that comes in varying weights, many of which are suitable for babywearing. However, in the US, "calico" refers to thin, printed quilter's cotton which is never safe to use for weightbearing purposes.