I remember in the World Cup in 98 them discussing Denilson's World Record move so he must have joined them in 98/99.
I had a great game with Betis years ago, but it may not have been ODB as I couldn't remember having Oli, but I did have Joao Tomas leading the line with Denilson and Joaquin firing balls into him from the wing.
Denílson was indeed later than CM97/98. Betis were always fun right through the CM2 and 3 series though.
I've actually got a save with them now on 00/01, but I took over them midway through the 3rd season so unfortunately only Ito and Prats remained of their team.