OTB applications are available from QGIS. Use them from the processing toolbox, which is accessible with Processing → Toolbox. Switch to ”advanced interface” in the bottom of the application widget and OTB applications will be there.
If QGIS cannot find OTB, the ”applications folder” and ”binaries folder” can be set from the settings in the Processing → Settings → ”service provider”.
On some versions of QGIS, if an existing OTB installation is found, the textfield settings will not be shown. To use a custom OTB instead of the existing one, you will need to replace the otbcli, otbgui and library files in QGIS installation directly.