Mottram finished fifth in 3min 48.98sec, a little over a second behind winner Daham Bashir of Qatar (formerly of Kenya) who won in 3:47.97, the fastest mile in the world this year.
Mottram's time surpassed the Australian mile record of 3:49.91 set by Simon Doyle in 1991, also in Oslo.
"It was a great time.... It couldn't have gone any better than a PB and a national record, but the big show's in a couple of weeks and that's what I really want."
Also in action for Australia was sprinter Joshua Ross in his last hitout ahead of Helsinki, clocking 10.28secs for sixth place, in a race won by Aziz Zakari of Ghana in 10.02sec.Subscribe to our newsletters to keep up to date with Athletics in Australia.