Use math.op to define things the same way predefined lim and sin for example are defined. It takes an argument to set limits.
Also note that you can force it directly using the limits function (scripts is the opposite). And in inline equations even lim has subscripts instead of limits, but it can be forced with limits again as shown below…
#let opA = math.op("opA", limits: true)
$ opA_x $
$ limits("opB")_x $
$lim_x$
$limits(lim)_x$
