What conditions determine who gets an organ transplant first?

Depends on organ: It really depends on organ to be transplantated. Some organs like kidney, depend mainly on time on the list, other organs depend on how sick you are (liver, heart).

Varies by organ: Factors are determined by national committees and available at the unos website. Kidney donor < 36 yrs: ABO type, waiting time, closeness of match, proximity, recip age < 19 kidney donor >35 yrs: ABO type, wait time, closeness of match, proximity liver:abo type, illness severity, proximity, size heart: ABO type, severity of illness, size, wait time lung: ABO type, size, severity of illness.