Shock-Tech uses O rings as well for their depress pin. They sold very high end paintball products as well as the ASA On/Off Valve. The O rings on them are also required to replaced after each weekend game or refill. The company didn't do too good in products and thus resulting in going out of business. Some stores will still sell their ST(Shock-Tech) products, but that doesn't mean just because you pay for higher end ASA On/Off valve that you wouldn't need to replace the O rings.
I believed if you want to go the super duper cheap way, your local paintball store can sell you 100-600 pieces for nearly $5-$8 dollars.
They are just something that needs to be replaced often. I would say it's best to change it every time you refill the tank. In my chase, it's 5-6 months. Even though it might be fine, it's just that extra piece of cents will further lessen prevent a leak for you.
She winks, Let's dance.