You may want to move OpenCPN, and the chart plugins (oeSENC, oeRNC), to the external SD card if not there already. (Be sure O has permissions to read/wright to that card.)
Make a chart directory on the Ext. SD card and move your charts. (Some Android versions don't accept files to be stored at a arbitrary location. In that case you can though still store in a directory appurtenant OpenCPN or its plugins like: ....Android/data/org.....opencpn/data/charts. E.g. oeSENC will suggest its own directory as default when installing charts.
Point to OCPN by options->chart files->Add directory where the charts are located. -> Ok/Apply
You know the upper three dots in the file manager are traveling back one step, do you? You need to get down to the root to enter up into the external SD card.