Is the book "Astronomy for O'levels" by Patrick Moore too outdated now?

No.

The fundamentals of astronomy have not changed in almost a century. If you were to read it, and learn it all cover-to-cover you would be way, way ahead of anyone who relied on sources like the popular media and recent Hollywood movies for their (mis)education.

Once you have mastered the fundamentals, and know a galaxy from a solar system, and a chondrite from a pulsar, you can then perhaps delve into the more speculative topics that have arisen since the book was written.

BTW: one of my early texts was a little pocket book written by Patrick Moore. I used it to learn all the constellations visible from my location, plus a brief summary of the basic principles of astronomy.

Cheers!

As this must have been written before 1988, when O levels were abolished in the UK, there will be parts that are outdated, but the basics will still be true. It will be a good start but don't rely on it, wonderful as Sir Patrick was. Anything historical will be valid, but of course it won't contain anything that was discovered in the last 30 years.

Which is the same with all science texts - new stuff gets discovered all the time, so you are always best off with what you can get that's newer.