I excised and sold (on the same day) 100K worth of ISOs, can I invest this amount in an opportunity zone fund and defer my taxes?

No, your exercise and sale is a "disqualifying sale".  It is ordinary income not a capital gain. If the transaction occurred in 2018, the bargain element amount should have been included in the total wages reported in Box 1 of your 2018 Form W-2.   Because this is a disqualifying sale (meaning you are disqualified from taking it as a capital gain and being taxed at the lower capital gains rate because you sold the shares less than a year after exercising the option).  You can only defer capital gains -both long and short term - by investing in Opportunity zones.  You cannot defer ordinary income.