In Yarn Weight what how do oz compare to grams?

[planetpapiermache]

This may sound like a stupid question, but it's helpful in buying yarn to be able to figure out how thick a piece of yarn is in order to compare it to another yarn and replace the yarn that's called for in a pattern. (And I know weavers have a different systelm of classifying the yarn with one number over another, but that's another question). The label will say something like this
150 yds [137m]/1 oz

But another yarn may only give yardage and grams
123 yards/50 grams

They are talking about the weight, right? And the grams are metric?
How do I convert these measurements again? These two yarns aren't equivalent weights, right?
The second yarn would be about 216 yards/ 1 oz, right?