ah I see so param 1 is a string.
If you want to control a slider by the parameter "lefty" I'd try to send normalized values to Reaper.
For this you'd need to normalize to the upper and lower bounds of "lefty".
This should control volume of track 1 (completely untested)
PHP Code:
min=-4000;
max=0;
oscmatch("/joyosc/devices/gc0/axis") ? (
oscparm(0,'s',#strParam);
!strcmp(#strParam,"lefty") ? (
normalizedOut=oscparm(1)/(min-max);
oscsend(osc_out,"n/track/1/volume", normalizedOut);
);
);