Is dedicated PreAmp with Oppo 105 needed?

Is there any way I can connect both, the Oppo 105 & Nakamichi RX-505 Cassette deck to the existing Halo A51 - 5 channel amplifier via Stereo only PreAmp to enjoy HT & Music both?

No- they have to be switched separately so that one source does not load the other.

@hitsofmisses Sound compromise is the name of the game.

If you don't want to compromise the sound so much, 2-channel equipment is the way to go. I would look into integrating it into your HT system; maybe look at a HT processor as an input to your 2 channel preamp and amp rather than a dedicated HT integrated amp; IOW using the amp and preamp to run your main speakers.

The reason is your HT will benefit from the improved performance of separates! 5-channel amps tend to run all 5 channels off of the same power supply, and that sort of thing is often audible as less authority and less detail.

The reason I would avoid running the Oppo directly into the amp is the volume control. Running it at lower volumes reduces resolution.

I would run a line section and use the Oppo with the volume up all the way, then use the volume control of the line section to set the right level. This technique works well- the improvement in resolution is easily heard.