My grammarbook says I can use /tretti(o)/, without /o/, although I haven't tried.
Stick to trettio with the O. To use it without the O looks very dialectal in text. Even colloquial writing will use the O.
where I lived (fairly far north) we didn't pronounce the "o" but always wrote it
do you guys say halvtimme for "half hour"?
The new voice doesn't pronounce the O in trettio. Is this a mistake?
No, "trettio" is most commonly pronounced as "tretti", unless you need to be very clear.