My husband is a part-time employee, he is supposed to work 30 hours a week. However, he frequently works more than 30 hours, much of the time he works over 40 hours a week. Is he supposed to be getting overtime pay when he exceeds either 30 or 40 hours? We live in NY, if that matters.

He is supposed to get time and a half if he works over 40 hours in a payroll week (unless he lives in, then it's 44 hours).

If he works over 40 in a pay period. (week) unless he is in some job area. ** may depend on what job he does, example commission sales or a police officer may not

True, federal law excludes some types of employees from the time and a half rule (commonly called "exempt" positions). The Fair Labor Standards Act, U.S. Department of Labor, Wage and Hour Division, outlines those occupations:
http://www.dol.gov/elaws/esa/flsa/screen75.asp

NY follows these exclusions, but DOES require that workers receive at least one and one-half times the minimum NY wage of $7.25 for OT.

When I was a officer at one location, it really was bad, we only got a hour of "comp" time for working over time, not even any pay at all. I had about 100 comp hours when I left them,

He is a mechanic working for the city.

Oh-and what should he do, should he ask for retroactive pay for the periods he worked over 40 hours? Where should he go with this?

He should go in and talk to the Human Resourse clerk and/or manager and ask them to explain his pay, and how overtime is treated.

And make sure it is in a pay period, some places figure pay like Wed though Tuesday, not Sunday though Sat.

First are we clear that he is an employee, getting a wage, and not a contractor getting paid with no taxes taken out?
If he really is on wages, then he should have course ask for OT.
I'm surprised that the city would overlook this.
(If he's paid on a 1099 basis, no, OT does not apply.)
Start writing down who he talked with and when and what was said...

Google NY Dept of Labor for a list of regional offices.

Yes, he is an employee, paid hourly.

OK, one more question, what is the pay period (week, 2 weeks, monthly)?
They may argue that he has evened out to 40/hrs/wk over the pay period.
They may be calling him 'exempt' hourly.

Neither mean that they can withhold OT, I believe. Given that he's working for the city (NYC, or another city in NY state?) I'm sure the NY Dept of Labor would be interested. He should ask his personnel dept first, and try to keep it polite. People do get 'let go' after situations like this, so be sure to document every step of the way. You would have a pretty good case.

It's a 2 week pay period. I did think of that, that they may be just "evening" it out, but that didn't seem right or legal to me... I'm debating whether he should pursue it just because he may get fired for "some" reason...

He could ask the NY Dept of Labor first, instead. Anonymously.