Incoming/outgoing octets are the number of bytes transferred on that interface in each direction. This is a continuous counter and it will roll over back to zero when it reaches a value of about 4 billion (2^32) unless you use the newer 64-bit counters that are at a different OID. Your monitoring program should be smart enough to detect this rollover and give you useful graphs even through that event.
The maximum number of packets per second will depend on the interface speed, packet size, and possibly the CPU of the system. As the packets get smaller, a larger percentage of what is on the wire is overhead (ethernet header, IP header, tcp header, etc.) A packet can be anywhere between 64 bytes (IEEE 802.3 minimum length) and 1500 bytes on a typical network. You can extrapolate the packets per second using the interface speed. You will need to consider that CPU time is used to generate these headers, and faster speeds will require more CPU. A saturated 1Gbit/sec NIC will consume significant CPU resources.