I remember there being some debate last year about whether or not the 6 Plus uses Optical Image Stabilization during video capture, specifically because it was only listed under iSight camera features and not video features. However, looking at the tech spec page for the 6s/+ OIS is now listed under video recording features. In fact there seem to be a number of new listed features under video (other than just 4k of course).
The new or modified features:
http://www.apple.com/iphone-6s/specs/
http://www.apple.com/iphone-6/specs/
The new or modified features:
- Optical image stabilization for video (iPhone 6s Plus only)
- Slo-mo video support for 1080p at 120 fps
- Time-lapse video with stabilization
- Cinematic video stabilization (1080p and 720p)
- Improved noise reduction
- Take 8MP still photos while recording 4K video
- Playback zoom
http://www.apple.com/iphone-6s/specs/
http://www.apple.com/iphone-6/specs/