UK's OneSavings Bank reports robust lending in first quarter

* Q1 net loans and advances grew by 455 mln stg

* Sees (Shanghai: 600481.SS - news) net interest margins ahead of expectations

* Increase in new business opportunities at attractive margins - CEO

* Top performer on FTSE All Share Financials Index

* Stock gains nearly 6 pct (Adds details, analyst comments, share movement)

By Aashika Jain

May 6 (Reuters) - OneSavings Bank Plc (LSE: OSB.L - news) , Britain's new specialist lender, said first-quarter net loans and advances grew by 455 million pounds ($693 million), boosted by its purchase of a portfolio of UK-based second-charge mortgages.

Shares (Berlin: DI6.BE - news) in OneSavings Bank, which offers residential mortgages, buy-to-let and SME loans, rose nearly 6 percent in morning trade. The stock was the top performer on the FTSE All Share Financials Index.

The lender said it was confident of achieving net interest margins ahead of expectations, buoyed by the absence of competition in its main markets.

Shore Capital analyst Gary Greenwood said he found OneSavings in an advantageous position relative to the mainstream banks and also the mainstream "challenger" banks.

"OneSavings lends in more specialist areas, so their ability to defend margins and spur those margins is greater than the mainstream lenders," Greenwood told Reuters.

OneSavings, which saw growth in lending backed by a recovery in the buy-to-let market, said the housing market remained supportive of buy-to-let mortgages.

"We have seen an encouraging increase in new business opportunities at attractive margins and our capital position remains solid," OneSavings Chief Executive Andy Golding said in a statement.

Golding said that the bank was not experiencing pressure on margins despite an uncertain political backdrop.

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Barclays (LSE: BARC.L - news) raised its target price on OneSavings stock to 360 pence from 300 pence, and maintained its "overweight" rating.

OneSavings' peers Aldermore Group Plc (LSE: ALD.L - news) and Shawbrook Group Plc, which went public this year, joined the list of credible challengers to Britain's "Big Five" - Lloyds , HSBC, Royal Bank of Scotland (LSE: RBS.L - news) , Barclays and Santander UK.

Shares in Aldermore and Shawbrook fell marginally in morning trade on Wednesday.

"Until Lloyds, RBS etc. start to turn the taps on, we believe attractive volumes and margins will remain freely available to the specialist providers like OneSavings Bank," Numis analysts said in a note.

OneSavings, formed following the recapitalisation of Kent Reliance Building Society by U.S. private equity group JC Flowers in 2011, said it continued to see attractive opportunities for organic growth.

OneSavings Bank told Reuters in March that it was open to takeover offers and expected more consolidation among smaller banks after more than doubling its annual profit.

The stock was up 3.8 percent at 302 pence at 0950 GMT. ($1 = 0.6565 pounds) (Reporting by Aashika Jain in Bengaluru; Editing by Gopakumar Warrier)