I think if it was widespread in most of Europe for more than a few weeks it would have been spotted. Sequencing is usually patchy and slow in most countries but not that far behind. That said, it could easily have started in a country with high case rates but (politically driven?) low reporting transparency or one that does minimal testing or sequencing.
Countries like the UK will start chasing back through old tests though, but they're starting with the last two weeks (because they matter for contact tracing). In the UK, if it was widespread a month ago, we would have seen it - that doesn't mean it wasn't here, just that it wasn't here in large enough amounts to explain any case rate peaks here.
The virus theoreticians who look at every genome example and try to draw infection chains for cases suggest that, based on what they've seen so far, the first omicron case was probably in early October.