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I thought KLayout users would find this very helpful, on why .OAS format results in smaller file sizes and apparently some performance improvements as well:
I have personally switched to OASIS after Mattias pointed out that OASIS saves Layer names (in addition to layer numbers), and subsequently found that my file sizes would go from 150MB (GDS) --> 25kB (OAS), an even more important fact for transferring GB-level files. It's now my standard format, converting to GDS only on the writing tools that require it (Heidelberg for example).