1. Introduction

2. Observations and data reduction

3. Results and Analysis
3.1. Detected lines and spatial extent
3.2. OH emission line flux ratios
3.3. OH rotational temperature
3.4. Dependence of OH luminosity on source properties
3.5. Correlation of the OH flux with other species

4. Excitation and origin of OH
4.1. Spherical envelope models
4.2. Outflow model

5. Conclusions