Basically there is no need neither for software, nor for some special script. You can make with simple static pages a fake OTO which says it is OTO, but in reality user can navigate to find it again. At least user will be happy that he didn't lost the chance, and eventually he will use it to buy. And if he doesn't buy, then who cares?
OTO has been so much used and abused, that personally I don't believe in the "unique opportunity" which goes away. And usually, after bypassing the OTO, I encounter again the same - or better - discount offer or special upgrade ofer as "one more chance" later, on the membership page.
However if you really would like to have a genuine One Time Offer, I mean the OTO is intended to be really unique, and you have a good copy which gives a really reasonable and believable reason for the uniqueness of the One Time Offer, then you can simply google for [free php "one time offer" OR "OTO"], just remove the brackets, and you may choose some free goodies.
For instance in less than one minute I found on Google:
and
http://askmichel.icoder.com/2006/08/...timer/#more-24
Bingo! Just chose which script you like better, copy and paste it in your web-page code and you have what you desired.
I don't know personally Michel Komarov, the author of these scripts, but from reading his articles, I have the impression, that he is a good authority on this scripting subject.
You may find worth of browsing his site for more goodies to be used on marketing pages.
Regards,
Art Swerdstone
P.S.
If anyone feels too hard or boring using google search, I may offer one-two hours per day from my time for organic search on Google for just $50 hourly rate. Just PM me.