I am running a mac with OS El Capitan (10.11). I know I cannot use the disc version of TurboTax for tax year 2018, but will I be able to use the online version?

Yes you should    ... see the requirements here:    https://ttlc.intuit.com/questions/4479504-what-will-be-the-minimum-system-requirements-for-turbotax-mac-software-for-2018-tax-year

Current ONLINE requirements :  <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/1899861-turbotax-online-system-requirements-for-tax-year-2017">https://ttlc.intuit.com/questions/1899861-turbotax-online-system-requirements-for-tax-year-2017</a>

And you will be able to transfer from 2017 on your computer into the Online version.  So be sure to save and have the file ending in .tax2017 on your computer to transfer.