I've been granted a non-immigrant visa type O with Multiple entry by the Thai consulate in Sydney for one year because my son is studying in Thailand (non immigrant visa type ED )and we are in Thailand since June 25th ,we've a stamp on our passports  saying "admitted until 22/09/03 ".The thai consulate in Sydney told me that we just have to go to the immigration bureau in BKK to have a further 3 month stay (without border crossing) ansd so on for the year to come, but I'm a bit skeptical since I read in this forum that a border crossing is compulsory...

Can anyone in the same case tell me what to do?

You have to exit and enter every 90 days. Sydney is wrong.

A one-year extension in-country of your stay based on your son's study seems unrealistic to me. You have to do a border run latest on 22 SEP 03.

I am from the US and considering applying for a one-year O-A (retirement) visa. Money and other qualifications are not an issue. However, with this visa am I free to enter and leave Thailand multiple times during the year without checking in at Thai Immigrations (Suan Plu) or should I need to get a multiple entry non-resident visa or some other visa? I expect to travel out of Thailand at least once every quarter.

Thanks

You need to obtain a multiple entry re-entry permit from Immigration. That'll enable you to exit and re-enter during the year. A re-entry permit is a must.

I ckecked my visa and it's only written "M"for N° of entry

Do I need to ask for a re-entry permit if I cross a border

If yes ,is it at the immigration bureau?

One last thing : under the label remarks ,the "Extension of stay not permitted" has been striped ...Doesn't it mean that I could stay longer than 3 months (without border crossing) as the  Sydney consulate told me?

Do I need to ask for a re-entry permit if I cross a border

If yes ,is it at the immigration bureau?

Not unless you receive an extension of your 90 day stay by immigration inside Thailand.  As long as use your multi entry visa in 90 day blocks you do not need a re entry permit.  Believe the 'Dr' was answering another poster.

That 'can not be extended' phrase does not seem to have any real meaning as even if marked immigration can extend it.  Believe the old/real meaning that you could not apply for a 30 day extension as with a tourist visa (as you have already used up the full 90 days); but that is just a guess.

There is nothing lost asking immigration about extending as you were told; but be prepared to make a trip if it can not be done.