Teksel - it's not that in the OLB so the new Frisian word appears to be Germanised while the English have the same original letter sound as the Frisians should - that would be Texel.
The Frisian language then appears to have been Germanised at some point, while the English kept the older sounds the Frisians should still speak but they 'speak fancy now'.
English is very unfancy language, the fancy words have come in from fancy languages.
X marks the spot.
Teksel (pronounced the same as Texel) is indeed not in the OLB, there it's called "Texland". Texel is like an abbreviation of Texland. In fact - even though I am Dutch - I pronounce it with a -ks- , not with -ss-.
We've discussed this before, but just as a reminder a link to a Dutch wikipage: http://nl.wikipedia.org/wiki/Texla
And you will read Thesla ("Zuidland") as one of the (older) names for Texel, and it means "South Land" (remember we talked about it in relation to Otharus' post about Tessaloniki??).