Oastler was a star at the Premier League exit trials and sparked a 13-club chase for his signature.
But it's QPR who won the central defender's vote, awarding him a one-year professional deal.
He told the Portsmouth News: "Obviously I was upset after being released by Pompey, but there was no way I was going to give up.
"I'd been with them since the age of eight, I'd had a season ticket for eight years, I live two minutes from Fratton Park. I support them and I always will - they are my team. Unfortunately it wasn't to be, that's how football is, but I was determined not to let that stop me.
"There were 60 lads who went to the exit trials in Warwick and I knew I had no choice - I had to perform otherwise I would not have got anywhere. You only have two 40-minute games to impress in as well, but fortunately it worked out and 13 clubs came in for me.
"Glenn Hoddle rang me up personally - I thought it was my mates winding me up. But I wanted a pro contract and QPR were the first club that came in for me. I was upset when I was released from Pompey but I'm definitely happy now."