Apple has been using the Helvetica Neue font both for its desktops and mobile devices. Both OSX 10.10 and iOS 8 come with Helvetica Neue pre-installed as default fonts though that could well be about to change. According to a report in 9to5Mac, Apple is all but set to move away from the long term relationship with Helvetica and replace the font with San Francisco in iOS 9 and OSX 10.11.
The San Francisco typeface has been used on the Apple Watch already and largely has been credited to have been cleaner and sleeker font on smaller displays as compared to Helvetica. It is due to this easy scalability of the font that the decision has been made as the font scales dynamically clearly and legibly for all purposes.
However, the decision to change the system font will not come so easy as all the pre-bundled applications will need to be updated with the new font and not just this, even the third party applications will have to be updated to avoid any sort of eye sore, and given how strict Apple has been about the quality of the applications it hosts, you can be sure that there will be more on this move if it actually does happen. The report also outlines that Apple has been setting the wheels in motion for this change since March and that is the result why the new MacBook Air keyboard has characters which are written in the San Francisco font.