You can use what qualitative data analysis programs such as NVivo and MAXQDA call "mixed methods" analysis. This requires you export the O-E responses to the qualitative software, then code them as usual in the software, and finally export the codes back for merger with your quantitative data.
Alternatively,, you can do the same thing manually by printing out the responses, developing a coding system, and then going through your data one resondent at a time and applying the codes. Typically, this involves creating one variable per code, and then scoring those variables 1 or 0 according to whether a given respondent mentioned that code.
Qualitative data can be analysed using TPCA (transpositional cognitive approach) which is a method and analytical tool especially in phenomenological studies. The TPCA has six stages and sixteen analytical research steps. Similar to thematic analysis etc. but all involve the use of NVivo (computer aided software) or manually analysing the data.