Ojamashimasu is a greeting used when visiting someone's home in Japan. This also happens in Hololive, when your oshi is joining a collaboration in another members' channel, viewers also type it. Some members have different ways of saying it so everyone can recognize where they are coming from.
In Subaru's case, the viewers use Ojameri. A reference to a scene in Hanma Baki, where Hanma Yujiro enters a room breaking a bulletproof glass wall with his face. (chapter 86)