OCBC's planned takeover of Hong Kong's Wing Hang bank has prompted mixed reactions from credit ratings agencies.

Standard & Poor's was the first to issue its take on the news, saying on Tuesday that it was affirming its AA- long-term and A-1+ short-term credit ratings on OCBC.

It also affirmed its 'stable' outlook on OCBC's long-term credit.

OCBC had on Monday said that it was launching a $6.23 billion takeover bid for Wing Hang as it sought to deepen its penetration into the Greater China market.

"We affirmed the rating because we believe OCBC's financial profile will remain unaffected despite the bank's proposed acquisition of Hong Kong-based Wing Hang Bank," said Standard & Poor's credit analyst Chris Lee.

"We believe OCBC will continue to benefit from its strong market position and solid funding profile."

But the two other ratings agencies, Fitch Ratings and Moody's Investors Service, issued reports on Wednesday placing their ratings of OCBC under review, with a view towards downgrading.

For Fitch, the main worry is that a larger presence in Greater China might lead to higher risks for OCBC.

"The acquisition of Wing Hang Bank will increase OCBC's exposure to Greater China, where credit and operating risk is higher than in OCBC's home market of Singapore. This could potentially lead to deterioration in OCBC's credit profile," wrote Fitch analyst Mikho Irawady.

An acquisition of this size will also materially reduce OCBC's core equity Tier 1 capital ratio, he added.

"OCBC intends to raise equity to maintain prudent capital buffers after the acquisition, but even with the equity raising, Fitch expects that OCBC's core equity Tier 1 ratio will likely be lower than its pre-transaction level."

Moody's analyst Eugene Tarzimanov agreed, noting that the main reason for his review of OCBC's rating is the risk associated with the financing of such a large acquisition.

"Moody's understands that OCBC plans to finance the transaction through a combination of debt and equity. The success of the new capital raising is important for OCBC if it is to maintain capitalisation on par with similarly rated peers," he wrote.