Old School Runescape is a sandbox MMO based on Runescape circa 2007, which is to say that hand-holding is minimal to the point of non-existence: "You go through a five-minute tutorial which teaches you the absolute bare minimum and then you're unceremoniously dropped into the hometown of Lumbridge" is how Austin put it in his May review. Despite that (or because of it), it's actually quite good, and soon it will be playable on Android and iOS mobile devices as well.
All of this is relevant to us because the mobile versions will support full cross-platform play with PC, so you can keep on playing even when you're out and about: "You can just pick up from where you left off at home," Jagex said.
The mobile versions have been available for awhile now through closed beta tests and a soft launch in Canada, Denmark, Finland, Iceland, Norway, and Sweden, and will go into full release on both platforms on October 30. The good news is that if you already have a beta version installed, or if you live in one of the soft launch countries, you can keep on playing without interruption; the bad news is that if you don't, you'll have to wait, because moving the app into the preorder stage requires its removal from the Google Play store.
You can preorder or pre-register (I'm not sure what the difference is) Old School Runescape for Android devices here, or iOS here. Pre-release access is not available in some countries, "mainly due to our concerns over the experience players will have due to the lack of local worlds in those regions," Jagex said. You'll need iOS 10 or later or Lollipop 5.0 or later on Android to run it. Full details are available at runescape.com.