What is the "OP" designation at the end of a FlexiFit part number?

"OP" is short for O-ring Position. Housings with G1-1/4" connections rely upon an o-ring to seal within the socket. The OP is the length from the o-ring to the inner flange of the connection. The length should match the depth of the socket on the vessel.

Hamilton Flexifit BIO stainless steel inline sensor housing with knurled fitting and blue reference junction