From the Jargon File version 4.4.7:

Ob-: /ob/, pref. Obligatory. A piece of netiquette acknowledging that the author has been straying from the newsgroup's charter topic. For example, if a posting in alt.sex is a response to a part of someone else's posting that has nothing particularly to do with sex, the author may append ‘ObSex’ (or ‘Obsex’) and toss off a question or vignette about some unusual erotic act. It is considered a sign of great winnitude when one's Obs are more interesting than other people's whole postings.

In this context ObTrav implies the author of a post has wandered from the Traveller topic (admittedly a broad subject for which almost anything can be considered on topic), and wants to return to proper subject.