We investigated the main-line spectral evolution with shell thickness of oxygen rich AGB stars .
The study is based on a sample of 30 sources distributed along the IRAS colour-colour diagram .
The sources were chosen to trace the Miras with thick shells and the whole range of OH/IR stars .
The Miras exhibit a 1665 MHz emission strength comparable to that at 1667 MHz .
Even though the Miras of the study have quite thick shells , their spectral characteristics in both main lines attest to a strong heterogeneity in their OH shell with , in particular , the presence of significant turbulence and acceleration .
The expansion velocity has been found to be about the same at 1665 and 1667 MHz , taking into account a possible velocity turbulence of 1-2 km s ^ { -1 } at the location of the main-line maser emission .
An increase in the intensity ratio 1667/1665 with shell thickness has been found .
A plausible explanation for such a phenomenon is that competitive gain in favour of the 1667 MHz line increases when the shell is getting thicker .
There is an evolution in the spectral profile shape with the appearance of a substantial inter-peak signal when the shell is getting thicker .
Also , inter-peak components are found and can be as strong as the external standard peaks when the shell is very thick .
This trend for an increase of the signal in between the two main peaks is thought to be the result of an increase of the saturation with shell thickness .
All sources but two - a Mira and an OH/IR star from the lower part of the colour - colour diagram - are weakly polarized .
The strong polarization observed for those two particular objects is thought to be the result of perturbations in their shells .