op-amp offset and input capacitor
If I understand your schematic correctly, you have a 1 kHz oscillator, DC-blocked by 2 nF, feeding a 100K input unity-gain inverting buffer that is DC-referenced to 2.5V.

2 nF at 1 kHz is 80 Kohms capacitive reactance, so you are going to have some attenuation of your oscillator signal on its way into the buffer.

As somebody once said, you have to mind your Z's and Q's.

Try using a 1 uF nonpolarized capacitor instead and see what happens.  Better, dial down the oscillator amplitude, so you KNOW that your +2.5 VDC reference will always be bigger, and try about a 220-250 uF electrolytic capacitor, appropriately polarized.