The move on Sunday comes after the state reached its threshold of 2 million vaccines administered to people in vulnerable, hard-hit communities.
"We expected it -- it's almost anti-climactic at this point, but it's a step forward. It gives people the relief they've been waiting for," Orange County Supervisor Doug Chaffee said.
The red tier allows for many more businesses and organizations to reopen. For instance, retail stores can now allow for half capacity instead of 25%, and museums, zoos and aquariums could reopen for indoor activities at 25% capacity, as could movie theaters and restaurants. Meanwhile, gyms can operate indoors at 10% capacity.
Wineries, breweries and distilleries can reopen for outdoor business only.
SoCal gyms moving back indoors after move to red tier
Catrina Cafe in Seal Beach was just one business that welcomed back customers back inside Sunday morning.
Craig Lyon, the owner of the establishment, said loosening those restrictions is what so many people needed.
"It's just such a bright spot going forward," said the cafe's owner Craig Lyon. "This is the start, you know. Finally we're going where we need to go, everything is looking great. People are getting healthier, they're getting their vaccinations... I think businesses are going to feel better, our customers are going to feel better, everybody's feeling better."
Orange County CEO Frank Kim said he was "excited" about moving up to the red tier, but he also cautions residents to keep their guard up.
"People should be careful and I think most people in Orange County are smart enough to assess their level of comfort and safety," he said. "For me, I intend to go visit my mom and have a meal with her, which will be great."
San Bernardino County moving into red tier
Moving to the red tier will also allow the reopening of theme parks as early as April 1 -- including Disneyland in Anaheim and Universal Studios Hollywood in Los Angeles County -- at 15% of capacity, with in-state visitors only.
Disneyland earlier this week said it plans to welcome back visitors by the end of April.
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Los Angeles County also moved into the red tier Sunday, though businesses are not opening until Monday.
Riverside and Ventura counties, meanwhile, are expected to move up from the most-restrictive purple tier to the red on Tuesday, with businesses opening their doors Wednesday.
City News Service contributed to this report.