p0300 code

I have a 1999 chevy malibu and after changing plugs and plug wires (after doing manifold intake gasket change good thing i caught it in time no coolant got into the oil it was only a small leak on the battery side by the thermostat) i took it up to autozone code said p0300 could this be the fuel injectors? If so when my cousin was hooking up the fuel rails the o-ring from the tube that hooks up to the fuel rails fell off and we had to put another one on.i am thinking the o-ring is still in the fuel rail clogging things up is this possible and what can i do for this?

How does it run? No mention of any misfire issues in your post.

I have a 2000 with the P0300 code also,
I had a Snap-On code reader attached and cleared the codes and a week later the light was back on so did it again and it showed that #6 was misfiring. but not enough to set the light. I feel the misfire when I start the car first time in the morning and after 6 days the licht flickered and then the next time I started the car it came on and stayed I have 3 coil connectors rusted (but not #6) but this AM I had a lot of white smoke coming out of the tail pipe so I'm thinking its time for another intake gasket, I just got the car but its obvious that the gasket was changed at least once, 81K on the car so the intake bolts show being removed is why I say its been changed.
I will give my mechanic the car Monday nihgt so he can connect the reader Tuesday morning hen its cold and see what's up. any way I'll clean the coils and plug wire connectors. and check the torque on the intake bolts, am
nd hope its not a head gasket or worse. I will be posting what it takes to fix mine when it happens but I have some reading to do now.