Setting up Oracle Instant Client on a Mac can sometimes be a bit fiddly. Below are steps to install the client on a mac.
Download instantclient base and SDK files from Oracle
Type sudo mkdir -p /usr/local/instantclient/ into your terminal. This will move those zipped files into that folder path.
Then type sudo chown [yourlogin] /usr/local/instantclient/ into your terminal. Change [yourlogin] to your own username and you will asked to input your password.
All contents should now be in /usr/local/instantlcient/instantclient_18_1/ or ...instantclient[your_version].
Now type pecl install oci8 into your terminal.
When asked for ORACLE_HOME type instantclient/usr/local/instantclient/instantclient_18_1. Replace 18_1 with the version you are using.