"Nope - That o-ring serves a c
"Nope - That o-ring serves a couple functions.
Firstly, it provides a seal to help keep out moisture which helps prevent the driveshaft from rusting into the crankshaft
Secondly, it prevents the grease that should be on the driveshaft splines (you did grease it before you put it back together right) from migrating to the top of the driveshaft, which can cause excessive pressure against the crankshaft which can damage the lower crank bearing or seal.
Can you live without it until you next do the impeller (which should be every second year) - maybe - but for how long it takes to drop a lower unit, why take the chance..."