What type of applications will an O2 trim be built into the BMS logic? We are operating a high pressure boiler rated for 450kpph without O2 trim. What is the recommendation and what are the pros and cons?

O2 trim will assist with proper combustion and optimize emissions.

With proper tuning and with the addition of O2 trim, you should be able to see significant savings. For example, if because of better O2 control you were able to average an O2 level of about 2.5% at high load this would be an efficiency increase of about 2.78%. Assuming your heat input to the steam from the boiler were 1000 btu/lb and the efficiency increased from 82.57% to 85.35%, and you were burning No. 2 oil with a btu value of about 19,500 btu/lb you could expect to see a savings in fuel of about 139 gallons per day. At $1.30 a gallon this would be a savings of over $65,000 dollors per year. If burning Natural gas you may be able to drive O2 a little lower for an even greater savings.
Efficiency numbers were obtained from CIBOs Energy Efficiency Handbook.

Most Nat. gas boiler excess O2 readings can be trimmed out to 1% and even less, with proper tuning, burner configuration, register settings etc. If you have the luxury of active CO monitoring, an even lower excess O2 can be achieved, particularly if you can "park" the unit at 100% capacity.