There are two gains that receive beneficial treatment with Qualified Opportunity Funds. First, there is the gain that you are deferring into the exchange. Depending on how long you hold the second property, this gain can be deferred up to 10 years and 15% of the gain will be excluded from your income. The second gain is the gain on the currently property you are holding. This gain is eligible to be 100% excludable from your income after 10 years.
There are a lot more requirements for qualified opportunity funds then mentioned already. One of the biggest requirements is that the property you invest in that is inside of a Qualified Opportunity Zone must be of original use. This means that simply buying a rental house will not work. There is 1 way around the original use requirement. You must substantially improve the property purchased. This means if you buy a $150k home in a zone you must put $150k of improvements into the home for your property to qualify.
I have not looked into the ins and outs of the interplay with 1031 exchanges. My clients have not had that question yet. I know that some is addressed in the regulations, though.