Is Pseudo-Random Generator (PRG) in OpenSSL uniformly distributed? I need to know can we use PRG of OpenSSL as a PRF?
Yes, the Pseudo Random Number Generator (PRNG) in OpenSSL is Cryptographically Secure, which means it passes statistical tests, but as @Maarten Bodewes suggests in his comment, why not go one step further and use that PRNG directly, rather than through OpenSSL?

OpenSSL can use EGD (which stands for Entropy Gathering Daemon). It is a process that taps into configurable entropy sources in a given system to provide cryptographically secure random numbers. EGD is actually modeled after /dev/random in *Nix Operating Systems, so it would not be available in a Windows system, for example, but its function is the same as /dev/random which is to gather entropy for random number generation.

If you are creating an application in a *Nix system (OS X, Linux, etc) chances are that EGD is taking input from /dev/random, so your best bet is to use it. If you are in a different OS, then EGD is probably the way to go.