“But I've managed to do OK.”

You're a professional photographer. You're talking to someone who's not a photographer about your job. You've told them how difficult being a photographer is because of competition from the Internet and other photographers. But you are successful anyway. You say:

But I've managed to do OK.

manage to (do something)

To "manage to" do something is to successfully do something, even though it was difficult. A few examples:

Aaron managed to get us front-row tickets to see The Rolling Stones. Can you believe it!?

I was a horrible student. I just barely managed to pass 11th grade.

do (well / OK)

To "do well" means to get a good result. It's usually used to talk about how much money someone has made:

Their software design firm has done extremely well over the last 5 years.

When something has only gotten slightly good results, or you want to downplay results that were actually very good, you say it "did OK":

We did OK in the first quarter, but I think that our second quarter growth will be really great.