Yes, the opposite is worse off. "You will be worse off if you date that person" - you will have difficulty if you date that person

"As you have a cold, you would be better off staying at home today" - you should stay at home today "He would be better off if he studied harder and got a good job" - He would be in a better financial situation - have more money

I mentioned "worse". Thank you anyway!

From my understanding, it means in a better condition or place or situation. Or a suggestion to change a situation maybe? For example: "Since you are so unhappy in your current job, you would be better off getting a new job that you enjoy somewhere else." For worst off, I believe it means you are in a worse position than before. Like : "You are worst-off financially now than when you started your business"