if we want to have the app available offline on mobile device, is there any easy way to plan the development in advance so that some components in outsystems can be later reuse in the offline app?

UI cannot be reuse.
tables structure can be copied.
business logic? is there any option to make the function available for reuse? convert to some dll? 

or should we develop mobile app first and later reuse in outsystems, seems easier this way since platform support integration.

anyone has done this before? appreciate if can share some lights here.

ANYONE PLZ?

While you can do offline apps with the current version of the platform - I encourage you to look at the announcements from Next Step in May about the next release of OutSystems platform.  The mobile experience is changing - all for the better. offline will be easy. https://www.outsystems.com/nextstep/videos/