You'll have a similar tax burden whether sold in the market or purchased by OKE in the merger.
Don't know if they announce the proxy voting results prior to the meeting tomorrow or only during the meeting.
OKE doesn't pass along any taxes. If you take the merger deal, you'll simply have new OKE shares, and the $25/MMP share of cash to pay your tax bill.
But the tax slate is wiped clean and starts from scratch with any new OKE shares you'll have.