I won't say concrete evidence but "pretty reasonable to assume" evidence and rumours from a number of people who I believe.
Plus - Radio Oxford have alluded to it frequently. They wouldn't do so if he was just someone coming to a game.
I can't corroborate any of this below but these are some of the things I've heard. Some of it might not be true, but I've taken the no smoke without fire approach, as well as the obligatory pinch of salt, to all of this:
1) Players were introduced to Tiger some weeks ago and told he was the new owner. Backroom staff have been told nothing and feel totally in the dark.
2) DE is not directly (i.e. not physically there) involved in the interviews for new manager. Interviews have taken place with new MD Niall McWilliams, Dave Jones, and two Thais present.
3) The appointment of the new Academy Manager Dan Harris had to be approved/rubber-stamped by the Thais.
4) Many recent home games DE has not been present. Tiger has been. I saw him in the boardroom myself a month ago in a very smart dickie-bow.
5) DE has himself said he cannot fund OUFC to take next step to Championship, and is always open to investment. He's also not denied something is going on with the Thais re investment/takeover - something I think it's safe to assume (given his relative openness to making statements about his intents in the past) he absolutely would have done if it was all B*****s.