OKCash (and most other bitcoin-based cryptocurrencies) require berkeley db 4.8 to build, however, this version is deprecated and not found on most distro's repositories. Fortunately, we can compile it ourselves!
$ wget http://download.oracle.com/berkeley-db/db-4.8.30.NC.tar.gz
$ tar -xzvf db-4.8.30.NC.tar.gz
$ cd db-4.8.30.NC/build_unix/
$ ../dist/configure --enable-cxx
$ make -j4 # If this causes problems, decrease the number
$ sudo make install
STEP 4 Download okcash source
$ cd ~
$ git clone https://github.com/okcashpro/okcash.git
$ cd okcash/source
STEP 5 Compile!
$ make -f makefile.unix -j2 BDB_INCLUDE_PATH=/usr/local/BerkeleyDB4.8/include/
If this fails, remove -j2. If it still fails, ask me. This will take a while (~1h) so take a break.