Tara should tell the survivors at Alexandria about Oceanside because it is in her best interest to maintain trust within the relationships that she has left in order to survive. Trust is hard to earn back in any situation and is even harder when people are already in a stressful situation. I believe she will regret keeping secret something that involves the lives of the other survivors.
Tara should tell the survivors about Oceanside, but I get that she's been captured by a group of women who are afraid and originally wanted to kill her. TWD has been crazy and I'm still mad about Glenn . Anyway, it seems that Negan's army is a lot larger than the Saviors, so it looks like war is coming. Tara is now having to think about what it means to be human and humane.
While Tara may be cautious with whom she breaches this topic, she should be honest with her fellow Alexandrians about the encounter she had with the Oceanside community. She should do so at the very least so they know that this group is out there incase they were to encounter them themselves. It is better to keep everyone informed because secrets tend to come back to haunt peoples.
Alexandria needs guns and Oceanside has them. Tara's loyalty does not rest with Oceanside. Instead her loyalty rests with Alexandria. Until she escaped, the women of Oceanside were going to shoot her. The only person from Oceanside that she owes anything to is the girl that helped her escape. It is better to help the bulk of the people from Alexandria than to not go back on her promise to one person from Oceanside.