You can be forgiven for not remembering the details about Oceanside.
The Walking Dead only visited this all-female colony once, way back in November, in what was at the time the least-watched new episode of The Walking Dead since Season 3 (it's since been replaced by "Say Yes").
In that episode, "Swear," Tara (Alanna Masterson) washed up on Oceanside's shore after getting separated from the still-missing Heath (Corey Hawkins). She learned that this community used to be under Negan's (Jeffrey Dean Morgan) rule in another location, but after the Saviors killed all their men and boys older than 10, they fled in the dead of night and ended up here. The Saviors don't know they're there, and Oceanside's leader Natania (Deborah May) wants to keep it that way. They want to be left alone, and they protect themselves with a substantial arsenal.
Tara decided to leave and head back to Alexandria, and on the road home, she realized the Oceansiders escorting her out were gong to kill her. She fought them off with the assistance of Cyndie (Sydney Park), Natania's kind, brave granddaughter, who as payment for saving Tara's life made her swear not to tell anyone about Oceanside.
Tara didn't tell for a long time, but eventually the need for guns in the fight against Negan became too pressing and she told Rick (Andrew Lincoln).
So now, Tara is leading the Alexandrians back to Oceanside, where everyone wants her dead except for one person, who will now think she's a snitch.
There are three possibilities for how the Alexandrians' rendezvous with Oceanside will go:
1. They will be welcomed with open arms and showered with all the guns they need to fight the Saviors, their common enemy.
2. They will be shot on sight, since Oceanside doesn't want any visitors and has gone above and beyond devising methods to keep them out.
3. There will be some combination of the two, where there will be a skirmish at first until the two groups come to an understanding.
The most likely bet is option #3, since the Oceansiders for the most part are not unreasonable people. Natania won't want to go along with Rick's plan, but Cyndie might, if she can get over the fact that Tara is a rat fink. We'll see what kind of deal can be reached.
The Walking Dead airs Sundays at 9/8c on AMC.