In what situations will the OS kill the thread and how commonly do these situations occur?
The OS will not kill the thread, unless it is killing the process — Android does not do anything with threads you create yourself. If you are the foreground process, you will not be killed. The odds of Android killing the process within a few seconds of you losing the foreground (after onPause()) are miniscule. The documentation on process lifetime — what there is of it — can be found here.