My OA Mk 1 is a 1981 build, and was the model that in some respects launched the brand in western markets. Ed Monk Jnr designed it and a number of subsequent models. At the time the Mk 1 was aimed at the upper end of the market, but well priced. It was a solid build. Those of us with these old boats may say things like the electricals are an achilles heel. But then you also need to consider the impact of "DIY owner improvements" over many years, and it is likely the root cause of most complaints.
OA moved to larger boats, and only recently re-entered the owner-operator segment again. The fact that they are still going strong after more than 40 years says something.
I don't have detailed knowledge of their larger and more recent designs and builds, but my guess is that they'd be ranked upper midrange in terms of quality.
But bear in mind brands can vary over time. Some models are better than others. Brand and model specific forums and fecebook group could be very informative where they are not restricted to owners only, and you are able to join them.