StavrosK, I spent a moment saying both and we have been saying it your way.

We've been saying: po as in police, lis as in lisp

I have heard that a valid pronunciation of the Greek is also:

po as in hot (closer to the 'a' sound that 'o' can make) lis as in lisp

You'll have to let us know if that's right!

The "o" in "police" is long, you need a short one. The English pronunciation is what you would transliterate as "poulis" (can't write IPA on the phone, but it's a short o followed by a oo sound). In Greek, the o is as in "hot" in duration, but not in sound (not close to the "a' sound).

There's only one way to say it in Greek (the one I posted above), I suspect that the person who instructed you to pronounce it as in "hot" meant the duration. I can't think of an example of an English word with a pure "o" sound at the moment, I'm afraid. You're going to have to listen to my pronunciation above to get it right :-P "O" as in "hot" is fine, as long as you don't pronounce it too much on the "a" side.