System requirements is more than just the operating system, which this field is about.
Also, it throws me off a lot, getting confused with dependencies in my mind.
In general, "operating system" or similar wording is used in many places. For example:
- Softpedia (Uses "Runs On", and under the hood it is itemprop="operatingSystem").
- Download.com uses "Operating Systems".
- tucows.com uses "OS".
- softonic uses "OS".
- MajorGeeks uses "Requires".
- filehippo.com uses "Requirements" (but under the hood: program-technical-tab-os-title).