since you mean /opt , this is easily fixable , you can resize the opt partition easily , you just need a pc running linux , just use ubuntu live cd if you don't run linux .
download backup menu on your n900 , then reboot with the keyboard open .
you will get to a menu , connect your n900 to pc ,then press 'w' on the n900
after about 15 seconds all partitions on the internal emmc , will be available for use as mass storage devices .
on your pc :
start gparted ,
unmount all partitions on the emmc from inside gparted .
shrink the 27 GB partition (vFat) to something smaller let's say 25 GB , then expand the second partition (the ext3 one) to fill that empty space . then click apply , notice that this may take some time to finish .
after this is finished , disconnect your n900 from pc , then press 'q' on your n900 , it will reboot , close the keyboard to boot normally to maemo .
thats all .