Sure, the OK Boomer thing is cute and funny. After all, my generation can be a bit out of touch with the modern times. So say it to your parents, your aunt, uncle, grandparents (if they're actually boomers - they're more likely Silent Generation).
But whatever you do, don't say it at work. Seriously.
Boomers are, by definition, over age 40. That means they're protected from age discrimination by the Age Discrimination In Employment Act and state laws like the Florida Civil Rights Act. Saying OK Boomer at work can get you into hot water with HR and EEOC.
If you're saying OK Boomer to a coworker, that could be deemed age-based harassment. If the boomer isn't amused, they'll report you to HR and you'll probably get written up or disciplined. Don't assume that the boomer in question is amused just because they're laughing with you. They may be laughing because they're embarrassed not to show they're being a good sport. It could still be offensive to them and therefore illegal harassment even if they're playing along.
If you're saying OK Boomer to a supervisor, all I can say is OK Millennial. That's going to get you fired. Don't make fun of your boss, and especially don't make fun of them for being old and out of touch.
If you're saying it to a subordinate, you have just given them direct evidence of age discrimination. If you fire, demote, deny a promotion, or even write them up, they can possibly prove that you did it because of age using that comment as evidence. I'd add it to the list of ageist things not to say like asking when they're going to retire (unless you have an actual need to know for reasons other than replacing them), calling them old man or old lady, saying the company needs a younger image, saying they're senile or ancient, and other age-related comments.
Interestingly, it probably doesn't go the other way. If a boomer says OK Millennial, that might not be age discrimination. Millennials haven't turned 40 (yet - they will start turning soon) so most age discrimination laws don't cover discrimination based on being too young. You might get crosswise with a company anti-bullying policy though, so it's probably best to avoid saying it at work.
Bottom line: don't insult colleagues based on their age.
OK Millennials and Boomers. Back to work.
Have a general question about employment law? Want to share a story? I welcome all comments and questions. I can't give legal advice here about specific situations but will be glad to discuss general issues and try to point you in the right direction. If you need legal advice, contact an employment lawyer in your state. Remember, anything you post here will be seen publicly, and I will comment publicly on it. It will not be confidential. Govern yourself accordingly. If you want to communicate with me confidentially as Donna Ballman, Florida lawyer rather than as Donna Ballman, blogger, my firm's website is here.