Surface magnetic fields are detected in less than 10% of the massive O-type star population and even less frequently among "old" massive stars approaching the terminal-age main sequence (TAMS). It is unclear to what extent the rarity of magnetic detections in massive stars near the TAMS is due to magnetic field decay or observational biases. We report the detection of a weak surface magnetic field in the O-type giant 63 Oph (Teff = 35.0 ± 0.3 kK, logg=3.51±0.03 ) from new ESPaDOnS circularly polarized spectra. The mean longitudinal field strength associated with the magnetic detection is <Bz> = 84 ± 14 G, which we use to set a lower limit on the dipolar field strength of Bp ≥ 300 ± 50 G. We report Balmer line-core equivalent widths (EWs) and radial velocity measurements from the analysis of spectra primarily obtained by the IACOB project with the FEROS, FIES, and HERMES spectrographs. We identify a dominant period of ∼19.8 days in the EWs, which we attribute to the effects of a rotating magnetosphere under the oblique rotator model. We do not identify any coherent signals in a time series analysis of archival Hipparcos, ASAS-SN, and K2 photometry. Our findings show that 63 Oph may be a rare link between strongly magnetic massive stars detected on or near the zero-age main sequence and weakly magnetic O-type supergiants. Additional observations are needed to fully constrain 63 Oph's magnetic field geometry and magnetospheric properties.