The core for the open cad platform OpenCascade can be a bit tricky to install on Linux due to the need to compile it yourself before installation (many different third party dependencies). Therefore a short step by step description for this can be found here for a clean installation of Ubuntu 18.04.
Start by opening the terminal in Ubuntu and installing general system packages needed:
sudo apt-get install software-properties-common sudo apt-get install libtool autoconf automake gfortran gdebi sudo apt-get install gcc-multilib libxi-dev libxmu-dev libxmu-headers sudo apt-get install libx11-dev mesa-common-dev libglu1-mesa-dev sudo apt-get install libfontconfig1-dev
The minimum requirements for third party dependencies to run OpenCascade itself is Freetype 2.5 and Tcl/TK 8.6.
Edit: To install Freetype 2.5 just run
sudo apt-get install libfreetype6 libfreetype6-dev
Tcl/TK 8.6 also comes as apt packages in Ubuntu 18.04 so they can be installed easily.
sudo apt-get install tcl tcl-dev tk tk-dev
Download the OpenCascade 7.4 source from the download center and place in the Downloads folder. Create a temp build folder and start the cmake wizard. Inspect suggested variables (shoud be prefilled) and start configuration with key c.
cd ~/Downloads tar xf opencascade-7.4.0.tar.gz mkdir tmp cd tmp ccmake ../Downloads/opencascade-7.4.0
Finish up with build and install
sudo make sudo make install